_______ caused the accident is still a completemystery.

People are often at a loss when frequently seeing smog level predictions ________ each other in different official forecasts.

Mary said that once ________ him you would like himand make friends with him

Jack, I haven’t seen your uncle for a long time.—He ___ Tibet as a volunteer because of the earthquake.

I don’t know if his uncle ________. — I think he ________ if it doesn’t rain .