根据短文内容,从短文后的选项中选出能填入空白处的最佳选项。选项中有两项为多余选项。
There are thousands of coral reefs(珊瑚礁) in the world; Unfortunately, however, they are now in serious danger. More than one-third are in such bad shape that they could die within ten years. 16 Here are the three main causes discovered.
The first cause is pollution on land. The pollutants run with rainwater into rivers and streams, which carry the poisons into the ocean. 17 So they will be easily infected(感染) with diseases.
18 Higher ocean temperatures kill the important food source(来源) for the coral—the algae, the tiny greenish-gold water plants that live on coral. 19 This process has happened more and more frequently in recent years.
