19.The discovery of new evidence led to___
a.the thief having been caught
b.the thief to be caught
c.catch the thief
d.the thief being caught
如何做此題？

20.-would you have helped her had it been possible?
-yes,but I __ busy with my work.
A,was b.had been c.have been d.am
時態怎么看？

21.who did the teacher as well as the monitor___an article for the wall newspaper?
a.has write b.has written c.have write d.have written
時態怎么看？

19 D
lead to里to是介词，所以后面跟动名词，且the thief与catch是动宾关系，所以用被动形式，但是lead to发生在catch之前或者说至少是同时，所以不能用完成形式。其中the thief是动名词的逻辑主语，与动名词一道构成动名词的复合结构。

20 A
上文的句子是一个对过去情况作出假设的虚拟条件状语从句，其中从句部分运用了倒装省略结构，根据动词时态形式可以判断出是对过去情况的虚拟，所以下文应该基于这个时间基点，用一般过去时。

21 C
这里用的是have sb do sth.的结构。
have是使役动词，who是have的宾语而不是主语，宾补要用不带to的动词不定式。