—You look unhappy. Did you fail the tests? —I ________ the written papers but didn’t pass the oral examination.
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.
However, at times this balance in nature is ______, resulting in a number of unforeseen effects.