C
定语从句,situation是先行词,原句应为为english beginners fail to use this expression (in the situation)麻烦再问一题the reason__they failed was __they didn't get fully prepared for the experiment。A.which,because B.which,that C.why,that D.why ,becauseC前面是定语从句，后面是同位语从句为什么第二个不填because呢同位语从句中用that连接句子，that无意义