The fact has worried many scientists ______ the earth is becoming warmer and warmer these years.
An increasing number of people enjoy sun-bathing, but experts warn that too much ________of the body to strong sunlight is harmful.
----Thank you for the trouble you ______ to fix my computer.----__________!
Guess what? I ran into a good friend of mine from _______ I was at kindergarten just now!
________ you kept your mobile phone on in such an important exam? It’s against the school regulation.
He is a man of few words, but he ______ be talkative sometimes.