As is often pointed out, knowledge is a two-edged weapon which can be used ________ for good or evil．

He’s only 17 and____not eligible to vote.

I shall take you back to Beijing _____ you are well enough to travel.

All too _____ it was time to go back to school after the glorious summer holidays.

A society cannot be successful if it throws tradition away, but it cannot be successful ____if we do something to stop progress.