工作倦怠又称职业倦怠，它是指由（）构成的一种生理上、心理上多维度的综合性症状。

A.情感衰竭

B.精神颓靡

C.去人性化

D.个人成就感降低

. As a teacher, he has never given his students so difficult a problem ____ they can’t work out.

A．if

B．that

C．when

D．As