Chemical pollution will bring about a threat to agriculture and food chain, and ________ to human health.
The radiation amount of a mobile phone for two half-hour periods per day is equal to ___ to an X-ray for ten seconds.
If only he _____ to send that letter to my grandpa, my grandpa will come to help me. Look, I’m so helpless now.
______ he realized it was too late to return home.
Several of the off-shore islands are ______ , now that we have a motor boat.
It’s already 10 o’clock, I wonder how it ____ that she was two hours late on such a short trip.