B. Valves are present in veins to prevent backward flow of blood.
The blood is carried back to heart through veins against gravity. But the blood pressure in veins is very low (compared to arteries). Thus there is possibility of backward flow in veins. To prevent this backward flow, there are valves in veins which allow movement of blood only in one direction-towards the heart.
c. De-starching a plant before photosynthesis is necessary because if we don't de-starch it, there will be starch in the leaf. Even if photosynthesis doesn't take place, we will still get the result as photosynthesis took place because there is already starch present there.
d. While taking food, the epiglottis closes the wind-pipe and food goes through food pipe. If we talk while eating, we will be distracted and epiglottis will not close wind pipe. Food might go into wind pipe and trachea will be choked.