They have got lost, or had an accident.
. 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.