一、
1.Swimming pools require plenty of care and maintenance in order to keep them _____ (function) properly.
2._____ (tell) many times how to do it, he still couldn't understand, so I had to do it myself.
3.The big earthquake hit the area several days ago, _____ (cause) the death of some people.
4.The room is empty except for a bookshelf _____ (stand) in one corner.
5.Standing on the top of the mountain is an ancient tower _____ (date) back to two hundred years ago.
