19. What is the distinct characteristic of invertebrates?
A. Presence of backbone
B. Absence of backbone
C. Presence of cranium
D. Lack of the cranium
20. Why do some mollusks have shells?
A. To make them move.
B. To protect their bodies.
C. To make it easier to find food.
D. So, they can live both on land and in water.
21. What are things needed by plants to make their own food?
A. Oxygen and chemicals
B. Oxygen and carbon dioxide
C. Water, chemicals, and oxygen
D. Carbon dioxide, soil, and sunlight​