When it ________ air pollution, Beijing faces the three “C”s, cars, coals and construction, which lead to Beijing’s thick air pollution.
Polly’s mother was anxious about ______ she could get home safely in ______ foggy a day.
Polly observed that she ______ by a tall man in a dark coat when she ______ the faces around her on the train.
______ is amazing to us is ______some plants can sense the soft touches of the insects on the leaves.
______ wants to enter the company must show his ID at the entrance.
I’ve heard a whisper ______ David and Alice are going to get married.