when he _____something useful,he wasted his time drawing lit

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when he _____something useful,he wasted his time drawing little pictures.
a.must have done b.shall be doing c.should be doing d.should have been doing

D
should是情态动词,表推测.
have been doing是现在完成进行时,表示从过去到现在一直在做某事.
情态动词should与动词原形连用时表示“应该做某事”,与进行时连用时表示“应该正在做某事”,与现在完成时连用时表示“本应该做了某事”（实际上没有做）,与现在完成进行时连用时表示“本应该一直在做某事（实际上不在做）”.
句意：（以前,）当他在本该做有用的事情的时候,他却浪费时间画小人画.
wasted是过去式,排除A、B.
C项：should表推测,应该.he should be doing something useful意思是“他应该在做某件有用的事”,这与后面的事实相矛盾.排除C.

when he _____something useful,he wasted his time drawing lit He always p_____ drawing pictures in his free time. He showed his （）（able）in drawing when he was quite young 词形变 He is getting ready for his drawing _____to playing computer games,he wasted lots of time he sho He started drawing when he was three yeas old,and when he is When he was young,he used to spend his ______ time collectin Mark Twain left school when he was twelve. Though he had lit He likes drawing,____art is his favorite subject. He shook his hesd from time to time when questioned about hi pressed from his parents,and( )that he has wasted toomuch ti Pressed from his parents, and _that he has wasted too much t