When Chinese people talk about American cities, the first_____ comes into their minds is San Francisco.
His words puzzled me so much that I had a hard time trying to ______ what he really meant.
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.