第二部分选择填空(共20小题;每小题1分,满分20分)21.—Have you seen _______ pen? I left it here this morning.—Is it _______ black one? I think I saw it somewhere.
—Where’s my driving license? Maybe I left it at home. —My Goodness! I ______ you would always take it along.
Though the training cost a lot of time and effort, I do think it ______.
After careful consideration, he decided to take the position, ______ he knew it would be difficult for him.
The teacher handed our writings back to us, ______ some necessary changes.
You must ______ that the power is off before you leave the lab.