_______ is often the case, we have worked out the producion plan.
A. Which B. When C. What D. As
请详细写出解题的过程和思路。

答案D.
As is often the case常是这样,这已和成语一样的常用语了,作状语用.
as is often the case..
情况常常如此
更多例句:

1.Haste makes waste, as is often the case.
忙中有错是很常见的。
2.He shouts when he gets angry, as is often the case.
他生气时就大叫，这是常有的情形。
3.As is (often) the case, Mary was late for school again.
就像平常一样，玛丽又迟到了。
4.As is often the case, humans are the primary cause of extinction.
通常情况下，人类是物种灭绝的主要原因。