The rivers will become dirtier and dirtier ______ we take action to protect them.
I was doing my homework________my father was reading newspapers. A.how B.while C.if
—I was at the cinema at 9 o’clock last night.What about you? —I________at home.
—Do you know________? —This afternoon.
We were________surprised that nobody believed that it was true.
The bridge________by the villagers last year.