问题补充：
China is embarrassed by such a fact: Chinese visitors rubbish on the Great Wall, followed by a foreigner picking up the litter.
A. will drop
B. would drop
C. are dropping
D. had dropped
答案：
A. will drop
B. would drop
(答案→)C. are dropping
D. had dropped
解析：句意：中国为这样一个事实感到难堪：中国的游客正在长城上扔垃圾，后面跟着一个外国客人在捡拾这个垃圾。表示动作正在发生用现在进行时。will drop是一般将来时，表示动作发生在将来；would drop过去将来时，表示在过去某一时刻看将来会发生的事情； are dropping现在进行时；had dropped过去完成时，时间上是过去的过去。 根据语境判断选C。
考点: 考查动词时态的用法。
