As soon as his teacher begins lecturing, he begins to doze off, not intentionally but _____.

A model or an experiment in a science class can help us see things which would       remain hidden．

I’m worn out,____,the bike doesn’t work well,___ I can’t go further.

________ they heard the shout for help，they rushed out.

There was such a long queue for the ticket that I ________ gave up, though I wanted to attend the concert so much.