His sister left home in 1998, and _________ since then. 

I      to go for a walk,but someone called and I couldn’t get away.

—How many times have you been to China?—In fact, it is the first time that I__________ here.

We are now in the age of great changes. Everything sometimes ________ before it is felt.

In order to find the missing boy, the young couple            all they can over the past twenty-four hours.