what a pity 'tis nearing dusk一句中的“'tis”何解?在英语中类似省略（用法）还有哪些?谢

关于英语改写句子的题目
改为复合句：
1.many animal species are nearing extinction.they are protected by law.
2.bats are not birds.they are not insects.
3.you have no excuse for missing the meeting.you chose to set the time for eight last night.
4.i like dr.smith very much.math is now one of my favorite courses.i work hard at it too.
5.mary sings a song.jack plays the piano.
6.the film was over,and then we left.
改为简单句：
freda worked in a car wash,and she hated it.
the time was midnight,and the boys came back from a ***.

They are sitting()to the window A.closing B.near C.nearing D.closed选B,为什么?

they are sitting （ ） to the window Acloseing Bnear cnearing dclosed