3.(2019重庆渝中联考)It is required that all the candidates 　　    (hand) in their paper before 11 O'clock.

答案　(should) hand　句意:所有的参赛者被要求在11点之前上交试卷。 本句中主句有re-quired, 所以从句应该用虚拟语气,谓语动词用(should+) 动词原形。

4.(2018北京海淀期中,32)The price on this packet is wrong. It 　　     be$2.50, not $3.50.

答案　should　此处表示“应该”,所以用should。