4)
Coral reefs form in warm, shallow
ocean waters. A reef begins with tiny
sea animals called polyps. A coral polyp
builds itself a hard, cup-shaped
skeleton to protect its soft body. The
skeleton is made of limestone, a kind
of rock. When polyps die, then new
polyps grow in the same spot and leave
behind more skeletons. The stony
material from the skeletons builds up
over many years to form a coral reef. In this way, the reef grows larger and
larger.
What is the main, or central, idea of the passage?
Coral polyps have hard skeletons, which protect their soft bodies.
A coral reef is formed by the buildup of polyp skeletons over time.
A coral reef grows and changes over a long period of time.