Directions: For each of the questions below, ONE or MORE of the response(s) is (are) correct. Decide which of the response(s) is (are) correct. Then choose:

Let's go through these multiple-choice questions.

SECTION III

21. In a food chain:

1. Producers are autotrophic organisms. (Correct)
2. Each organism has many food sources. (Incorrect, this is more characteristic of a food web)
3. Producers release carbon dioxide during photosynthesis. (Incorrect, they take in carbon dioxide and release oxygen during photosynthesis) Only statement 1 is correct. The correct option is D.

22. During the bending movement of the arm:

1. The biceps muscles contract. (Correct)
2. The triceps muscles relax. (Correct)
3. The ulna and radius straighten. (Incorrect, the bones at the elbow joint bend) Statements 1 and 2 are correct. The correct option is B.

23. Myopia (short sightedness):

1. Occurs as a result of the eyeball being large. (Correct)
2. Near objects are seen clearly. (Correct)
3. Is corrected with the use of concave lenses. (Correct) Statements 1, 2 and 3 are all correct. The correct option is A.

24. In temperature control during overheating:

1. Sweating is inhibited. (Incorrect, sweating increases)
2. Vasodilation occurs. (Correct)
3. The hair erector muscles relax. (Correct) Statements 2 and 3 are correct. The correct option is C.

25. Birds fly easily because:

1. They have wings. (Correct)
2. They have hollow bones. (Correct)
3. They have streamline bodies. (Correct) Statements 1, 2 and 3 are all correct. The correct option is A.

SECTION IV

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 true but does not explain why glucose is partially broken down. The partial breakdown is due to the absence of oxygen.
The correct option is B.

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 explains why arteries need thick, elastic walls and a small lumen (to withstand high pressure as blood is pumped away from the heart).
The correct option is A.

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 explains why the Amoeba needs a contractile vacuole (to expel excess water that enters due to osmosis from the less concentrated environment).
The correct option is A.

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 explains the source of the gases, not how they cause acid rain (the chemical reactions involved).
The correct option is B.

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 explains the primary function of fur/hair in mammals, which is thermoregulation.
The correct option is A.