He was puzzled by this phenomenon. He began to try every approach _________ the best way of working it out.
.I like getting up very early in summer. The morning air is so good ___ .
.___ is known to us all is that the 2010 World Expo will take place in Shanghai.
.We decided to turn to ___ can help us out.
.--- What do you think made Mary so upset? --- __ her new bicycle.
.They ___ out because the light’s on. Let’s go upstairs and have a look.