The newcomers still don’t know ________ the difficult situation after practicing for 2 months.

下列各句中，加横线的成语使用恰当的一句是（ ）

Listen! There is a knock on the door. Who _____ it be?—It must be Jack. Just now I sent for him.

don’t believe these people have achieved success __________. They must have put much effort into their work.

you practice, youcan play the violin.