6.What is the topic of the text?A. What an atoll is.
B. How an atoll is formed.
C. How an island appears.
D. Why coral reefs rise higher.
E. How volcanic eruptions occur.


7. What happens when the waves break the reef
continually?
A. A lagoon is formed.
B. The volcano sinks.
C. The coral reef rises.
D. An undersea volcanic erupts.
E. Corals build an encircling reef.


8. What does the middle atoll look like?
A. A large shallow sea.
B. A deep hole in the ocean.
C. An islet with encircling reefs.
D. An island with sand and other materials.
E. A channel that links the central water and
ocean.


9. "As the reef itself crumbles, sand and other
material piles on top ...." (Last paragraph)
What is the closest meaning of crumble'?
A. Arise.
B. Float.
C. Spout
D. Collapse
E.Disannear​