Teaching is a difficult job ______ requires great skill and patience．
A．which B．where
C．who D．why
Drivers must stop when the traffic lights _____ red．
A．turn B．will turn
C．turned D．were turning
There was an excited expression ______ his face when he heard the good news．
A．of B．on
C．at D．over
Milk will turn sour soon on hot days ______ it is stored in the refrigerator．
A．since B．until
C．because D．unless
You'd better not ride the bicycle on snowy days; you ______ fall．
A．must B．might
C．have to
D．ought to
—Hi, Jack．You look a bit fat now．
—Yes, I ______ five pounds recently．
A．am putting on B．will put on
C．have put on D
．had put on
