teacher, he has never given hisstudents so difficult a problem ____ they can’t work out

下列词语中，没有错别字的一项是（ ）

Can I pay the bill by check—Sorry, sir. But it is the management rules of our hotel that payment be made in cash.

Why don’t you watch TV at 7 o’clock as usual—I am busy watch TV. I have to prepare for my English exam.

“_____ the election is over,” said Powell, “the time has come for me to step down as secretary of state and return to private life.”[ ]