I have been studying computer for years and I still ___

The little boy is dirty from head to foot because he ______ in the mud all morning

Mr. Smith, why are you always asking me to do the difficult work?—Because you _______ do such a thing and nobody else is fit for it.

—Why do you look so worried?—My computer broke down and my essay ________ unfinished since.[ ]

下列各句中,加点的成语使用不恰当的一项是（ ）