虚拟语气不解句The chairman requested that ___A.the members studied

虚拟语气不解句
The chairman requested that ___
A.the members studied the problem more carefully
B.the problems were more carefully studied
C.the problems could be studied with more care
D.the members study the problem more carefull
答案：D
请问本题为何选D?
本句是虚拟,后面句子应该用should do 形式,可是B也是should do形式吧?只不过是被动而已.