f there is no scientific proof secondhandsmoke causes cancer, there is no reason why nonsmokers should be forced to takethis risk.

My first summer vocation in the high school, _______I spent a fortnight on the farm, is really unforgettable

She worked deep into the night _____ a long speech for the president. [ ]

It suddenly_____ to me that I could use a computer to do the job.

Look! Is that man Mr. Li–No, it ______ be him. He has gone to America for a meeting.