The biggest challenge weall face is to prevent further environmental disasters. We ________ take action before it is too late.
句意：我们面临的最大挑战是防止进一步的环境灾难。我们必须采取行动，否则就太晚了。
考查情态动词。can能，会；must必须；may可能；might可能。根据“The biggest challengewe all face is to prevent further environmental disasters.”可知，问题比较严重，所以必须采取行动。故选B。
The biggest challenge weall face is to prevent further environmental disasters. We ________ take action before it is too late.
Listen up, guys!Everyone ________ wear a mask. Go and buy one if you don’t have one.
—Must I return this book before Friday, Sir?
—No, you ________,dear. You can keep it for two weeks.
You’d better take an umbrella when you go out, because it ________rain.
The man _______ be a doctor in this hospital, but I’m not sure.
—Whose notebook is this?
—It ________ be Alan’s. It looks like his.
Mary ________ go to London next year but she is not sure.
