An increased concentration of carbon dioxide and certain other gases in the Earth’s atmosphere ______ global warming.
Many of us have a strange sense that we do remember feeling as if we _________in a dream, but can’t quite recall the details.
By the time Jack returned home from England, his son _______ from college.
——Do you know where David is? I couldn’t find him anywhere. ——Well, he_______ gone far—his coat’s still here.
Do you think that bridge _________2,000 metres long? It doesn’t appear so long.
—If he______there were poisonous snakes in the mountain, he would not have camped there. —Luckily, he was found relatively quickly and_______to the local medical center.