When it______ air pollution, Beijing faces the three “C” s-----cars，coal and construction，which lead to Beijing's thick air pollution.
A. refers to B. comes to C. happens to D. speaks to
______ it have been the failure to pick up directions from the ground control that caused the air crash?
A. Must B. Could C. Would D. Should
The one concern for parents and students is ______ if the student pursues their university education in a foreign country, the student will not have the opportunity to establish social contacts in China.
A. what B. that C. whether D. /
Not until the train pulled into the next station ______ her suitcase gone.
A. Mary had found B. had Mary found
C. did Mary find D. Mary found
In class, fewer of the students who are not ___ their studies sometimes ____the dying robots,not listening attentively.
A. accustomed to ; are functioned as B. accurate in; function
C. sceptical of; are functioned D. addicted to; function as
Ladies and gentlemen, please ______ seated until the plane has come to a complete stop.
A. remain B. keep C. go D. get
