In a food chain: 1 Producers are autotrophic organisms. 2 Each organism has many food sources. 3 Producers release carbon dioxide during photosynthesis.

here are the answers to questions 21-30:

21) In a food chain:

1. Producers are autotrophic organisms. (Correct)
2. Each organism has many food sources. (Incorrect, this is not universally true for all organisms in a food chain)
3. Producers release carbon dioxide during photosynthesis. (Incorrect, producers take in carbon dioxide during photosynthesis and release oxygen) Therefore, only statement 1 is correct. D. If 1 only is correct.

22) During the bending movement of the arm:

1. The biceps muscles contract. (Correct, biceps are the primary flexors)
2. The triceps muscles relax. (Correct, triceps are antagonistic to biceps and relax during flexion)
3. The ulna and radius straighten. (Incorrect, they bend at the elbow joint) Therefore, statements 1 and 2 are correct. B. If 1 and 2 only are correct.

23) Myopia (short sightedness):

1. Occurs as a result of the eyeball being large. (Correct, or the lens being too curved)
2. Near objects are seen clearly. (Correct, this is the definition of myopia)
3. Is corrected with the use of concave lenses. (Correct, concave lenses diverge light to focus it on the retina) Therefore, statements 1, 2 and 3 are all correct. A. If 1, 2 and 3 are all correct.

24) In temperature control during overheating:

1. Sweating is inhibited. (Incorrect, sweating increases to cool the body)
2. Vasodilation occurs. (Correct, blood vessels widen to increase heat loss)
3. The hair erector muscles relax. (Correct, hairs lie flat to reduce insulation) Therefore, statements 2 and 3 are correct. C. If 2 and 3 only are correct.

25) Birds fly easily because:

1. They have wings. (Correct, essential for flight)
2. They have hollow bones. (Correct, reduces body weight)
3. They have streamline bodies. (Correct, reduces air resistance) Therefore, statements 1, 2 and 3 are all correct. A. If 1, 2 and 3 are all correct.

26)
First statement: During anaerobic respiration glucose is partially broken down. (True)
Second statement: Anaerobic respiration occurs in the cytoplasm. (True)
The second statement is not a correct explanation of the first. Partial breakdown is due to the absence of oxygen, not the location.
B. First statement True, Second statement True, and 2nd statement is NOT a correct explanation of the first.

27)
First statement: Arteries have small lumen and thick elastic walls. (True)
Second statement: Arteries carry blood away from the heart. (True)
The second statement is not a correct explanation of the first. The structure of arteries is adapted to the high pressure of blood leaving the heart, not simply its direction of flow.
B. First statement True, Second statement True, and 2nd statement is NOT a correct explanation of the first.

28)
First statement: Fresh water Amoeba possesses contractile vacuole. (True)
Second statement: The cytoplasm of fresh water Amoeba is more concentrated than the surrounding water. (True)
The second statement correctly explains the first. Due to the concentration difference, water constantly enters the Amoeba by osmosis, requiring the contractile vacuole to expel it.
A. First statement True, Second statement True, and 2nd statement is a correct explanation of the first.

29)
First statement: Acid rain results when Sulphur dioxide and nitrogen dioxide are released into the atmosphere. (True)
Second statement: Sulphur dioxide and nitrogen dioxide in the atmosphere originate from the burning of fossil fuels. (True)
The second statement correctly explains the first. The burning of fossil fuels is the primary source of these pollutants, which then lead to acid rain.
A. First statement True, Second statement True, and 2nd statement is a correct explanation of the first.

30)
First statement: The body of most mammals is covered with fur or hair. (True)
Second statement: Fur or hair assist in the control of body temperature. (True)
The second statement correctly explains the first. Fur or hair provides insulation, which is crucial for thermoregulation in mammals.
A. First statement True, Second statement True, and 2nd statement is a correct explanation of the first.