is known to us all is that the old scientist, for life was hard in the past, still works very hard in his eighties.
--You don't look so well, Mary. What's the matter with you? --Why?___________.
China has hundreds of islands, ___________ is Taiwan Island.
You must have waited here for a long time ,___________you?
Nearly________ of the surface of our planet is covered by water.
It’s said that __________ is going to give us a lecture.