阅读下面短文，掌握其大意，然后从21～30各题所给的A、B、C和D项中，选出最佳选项，并在答题卡上将该项涂黑。
Sea turtles have been on Earth for millions of years, but they may soon die out. The main 21 to them is people who kill them for food, according to Dr. Wallace J. Nichols of the University of California.
