C．how D．when

　　解析：选C　关于我们应该如何对待地球上的海和大洋。

　　15．A.calls on B．calls for

　　C．calls up D．calls at

　　解析：选A　call on sb. to do sth."号召某人去做某事"；call for"要求，需要"；call up"给某人打电话，想起某事物"；call at"拜访某地"。故答案选A。

　　16．A.long B．healthy

　　C．unhealthy D．open

　　解析：选B　本句话中活着与死亡相对，健康与污染相对。故答案应该是healthy。

　　17．A.national B．international

　　C．natural D．official

　　解析：选B　由本句中的World以及第三段最后一句可知应选international。

　　18．A.taken B．opened

　　C．joined D．held

　　解析：选D　2004年的环境日的活动在巴塞罗那举行而不应该是召开。故用held。

　　19．A.deep B．close

　　C．tight D．far

　　解析：选B　形容词close用来修饰名词collaboration"协作"。close"密切的"；tight"紧紧的"。in close collaboration with"与......密切合作"。

　　20．A.hoped B．wanted

　　C．meant D．was

　　解析：选D　be to do sth."将要做某事"。

　　Ⅱ.短文改错

　　Last month, our monitor suggested at a class meeting that we did something to decorate our classroom. Some classmates wanted to hang several pretty picture on the wall, while others thought this would be great to put some plants in the classroom. Most of us prefer the second idea. In the following week, variety of beautiful plants were brought to our class, which was immediately created a relaxing atmosphere. Every day after class, we devoted our spare time to care these plants, watering them regular. With fragrant flowers around ourselves, we felt in peace. But it surely helped us reduce some stress.

　　答案：第一句：did→do

第二句：picture→pictures; this→it