21. The number of smokers, _____ is reported, has dropped by 17 percent in just one year.
A
it
B
which
C
what
D
as
句意：正如报道的那样，吸烟者人数仅（过去）一年就减少了17%。as引导的非限制性定语从句位置灵活，可以置于主句的句首、句中或句末，指代的是整个一句话。而which只能置于句末，故选D项。
分析句子结构，发现考查的是as引导的非限制性定语从句
对非限制性定语从句关系代词which，as的用法不够了解。