16. Our students are free to study _____ interests them.
A. that B. whichever C. whatever D. anything
17. Mathematics _____ easy to learn.
A. is B. are C. can D. feels
18. It’s never _____ .
A. late enough to learn B. not late enough to learn
C. too late to learn D. so late as not learn
解析：活到老，学到老。too……to…… 太……而不能……, 这是一个否定句式，注意句子中有一个never，否定含义，双重否定，所以这个句子为肯定的语气。
19. I don’t know whether he is still _____ in collecting coins. He used to be, but that was several years ago.
A. interested B. interesting
C. interest D. disinterested
解析： 我不知道他是否还对收集古币感兴趣。他过去是这样的，但这是几年前了。短语be interested in doing… 对…感兴趣。
20. The noise outside was _____.
A. too irritating that
B. so irritating so his speech was hardly audible
C. so irritating that his speech was hardly audible
D. so irritating enough that his speech was hardly audible
解析：外面的嘈杂声是如此地令人厌烦，以至于很难听清他的演讲。 So…that… 如此…以至于… ； audible听得见的； irritating 让人气愤的，让人厌烦的。