. With mobile phones, it’s easier to communicate with people because you can __________them almost any time .

I plan to build an unusual house. It out of old things.

If you really have to leave during the meeting,you’d better leave ____ the back door[ ]

What actually ______ the accident has not yet been determined.[ ]

It was about 600 years ago _____ the first clock with a face and an hour hand was made. [ ]