一道情态动词的语法题分析,He should be wiling to work on his own,often at high speeds,but should be humble enough to consult others______his own knowledge not always prove adequate to the task in hand.A.should B.when C.because D.if为什么不能选其他三项,最好能说一下如果要选B/C/D 句子要怎么改,

一道情态动词的语法题分析,
He should be wiling to work on his own,often at high speeds,but should be humble enough to consult others______his own knowledge not always prove adequate to the task in hand.
A.should B.when C.because D.if
为什么不能选其他三项,最好能说一下如果要选B/C/D 句子要怎么改,