How do nitrogen fixing bacteria in the soil contribute to the nitrogen cycle?

25. Nitrogen-fixing bacteria convert atmospheric nitrogen gas ($\text{N}_2$) into ammonia ($\text{NH}_3$) or ammonium ($\text{NH}_4^+$). This process makes nitrogen available in the soil for plants. Nitrifying bacteria then convert ammonium to nitrites and then nitrates.

• 1. They change nitrogen into soil nitrates: Nitrogen-fixing bacteria initiate the process that ultimately leads to the formation of nitrates in the soil. This statement describes their contribution.
• 2. They return nitrogen to the atmosphere: This is the role of denitrifying bacteria.
• 3. They change ammonium to nitrates: This is the role of nitrifying bacteria.

Therefore, only statement 1 correctly describes a contribution of nitrogen-fixing bacteria.

The final answer is $\boxed{1}$

26.

• First statement: Glucose is completely broken down during aerobic respiration. (True. In aerobic respiration, glucose is fully oxidized to carbon dioxide and water).
• Second statement: The end products of aerobic respiration are more toxic. (False. The end products of aerobic respiration are carbon dioxide and water, which are generally less toxic than some byproducts of anaerobic respiration, such as lactic acid or ethanol). Since the first statement is true and the second statement is false, the correct option is C.

The final answer is $\boxed{C}$

27.

• First statement: The female Anopheles mosquito transmits plasmodium. (True. Female Anopheles mosquitoes are the vectors for the Plasmodium parasite, which causes malaria).
• Second statement: The female Anopheles mosquitoes feed on the blood of its host. (True. Female mosquitoes require blood meals for egg development). Both statements are true. However, the second statement describes a necessary condition for transmission but does not explain the mechanism of transmission of Plasmodium itself. Therefore, the second statement is not a correct explanation of the first. The correct option is B.

The final answer is $\boxed{B}$

28.

• First statement: Habitat loss is the most frequent cause of extinction today. (True. Habitat destruction and degradation are widely recognized as the leading causes of species extinction).
• Second statement: Global warming is expected to cause many extinctions in the twenty-first century. (True. Climate change, including global warming, is projected to be a significant driver of future extinctions). Both statements are true. However, global warming is a separate cause of extinction, not an explanation for why habitat loss is currently the most frequent cause. Therefore, the second statement is not a correct explanation of the first. The correct option is B.

The final answer is $\boxed{B}$

29.

• First statement: During photosynthesis light energy is converted to chemical energy. (True. Photosynthesis captures light energy and stores it in the chemical bonds of organic molecules like glucose).
• Second statement: Green plants combine carbon dioxide and water using solar energy to produce organic food. (True. This describes the overall process of photosynthesis, where light energy is used to synthesize organic compounds from inorganic precursors). Both statements are true, and the second statement provides a correct explanation of how light energy is converted into chemical energy during photosynthesis. The correct option is A.

The final answer is $\boxed{A}$

30.

• First statement: The cell wall in a plant cell is made up of chitin. (False. Plant cell walls are primarily composed of cellulose. Chitin is found in the cell walls of fungi and the exoskeletons of arthropods).
• Second statement: Cellulose provides roughage and prevents constipation in human diet. (True. Cellulose is a type of dietary fiber (roughage) that aids in digestion and promotes regular bowel movements). Since the first statement is false and the second statement is true, the correct option is D.

The final answer is $\boxed{D}$

31. The description "Destroys leaves of plants through the formation of acid in rain" refers to acid rain. Acid rain is primarily caused by the emission of sulfur dioxide ($\text{SO}_2$) and nitrogen oxides ($\text{NO}_x$) into the atmosphere. Sulfur dioxide reacts with water and oxygen to form sulfuric acid, a major component of acid rain.

• A. Carbon dioxide contributes to global warming, not acid rain that destroys leaves.
• B. Carbon monoxide is a toxic gas, not a primary cause of acid rain.
• C. Chlorofluorocarbons deplete the ozone layer.
• D. Sulphur dioxide is a major precursor to acid rain.

The final answer is $\boxed{D}$

32. The hormone that causes uterine contractions during labour is oxytocin.

• A. Testosterone is a primary male sex hormone.
• B. Oxytocin stimulates uterine contractions during childbirth and milk ejection.
• C. Oestrogen is a female sex hormone involved in the menstrual cycle and pregnancy, but not directly in causing contractions during labour.
• D. Progesterone is a female sex hormone crucial for maintaining pregnancy.

The final answer is $\boxed{B}$

33. A social insect lives in a highly organized colony with a division of labor. Among the options, honey bees exhibit this characteristic.

• A. Mosquitoes are generally solitary.
• B. Cockroaches are not social insects.
• C. House flies are not social insects.
• D. Honey bees live in complex social colonies.

The final answer is $\boxed{D}$

34. Tissue fluid (also known as interstitial fluid) is formed when blood plasma filters out of capillaries. During this filtration, most large