What are the three most populated countries of the world?

What evidence does the writer give to support his argument about coral reefs? a) he speaks of the activities of coral polyps. b) he describes the composition of the great barrier reef. c) he emphasizes the ways in which coral reefs are fragile. d) he points out the ways that scientists do research on the reefs.

One disadvantage that darwin faces today is w. enemy attacks x. natural disasters y. lack of land bridge z. high population density

How are the arctic and antarctic circles alike?