Tom's teacher and friend Mr. Smith.
The fact has worried many scientists_______ the earth is becoming warmer and warmer these days.
Seeing the happy ______ of students playing basketball there, I can’t help thinking of my school years.
There are so many viruses on the Internet. If ______ accidentally, the viruses may make your computer break down.
I _______ to your party, but I was prevented by the heavy rain.
---Where did you come across your old classmate Jane?---It was in the supermarket _______ she worked _______ I came across her.