Scientists have known for a long time that vitamin D is essential for humans. If children have a vitamin D or calcium deficiency, they can develop rickets, a softening of the bones. New studies are showing that people of all ages need vitamin D to help them fight off diseases by keeping their immune systems strong.

1. The main idea of this paragraph is that vitamin D _______.

a) is found in milk

b) has been studied by scientists

c) is no secret

d) is important for good health.