With such a variety of factors, ______ is no wonder that scientists don't fully understand the natural history of the disease.
A、
that
B、
it
C、
this
D、
there
【正确答案】
B
【答案解析】
考查代词。it为形式主语，真正的主语为that引导的主语从句。it is no wonder that...意为“难怪”。
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