例句：
When should you prune apple trees?
苹果树应该什么时候剪枝？
Should police officers carry guns ?
警察应该佩枪吗？
No income should remain undeclared.
一切收入均应申报。
The selectors should resign en bloc.
这些选择者应该集体辞职。
You should have seen it!
你应该看见的！
