4．有a、b为两个分运动，它们的合运动为c，则下列说法正确的是(　　)

A．若a、b的轨迹为直线，则c的轨迹必为直线

B．若c的轨迹为直线，则a、b必为匀速运动

C．若a为匀速直线运动，b为匀速直线运动，则c必为匀速直线运动

D．若a、b均为初速度为零的匀变速直线运动，则c必为匀变速直线运动

Reportedly, yesterday a group of American soldiers were walking along the road in Iraq when a bomb was _______, three of whom were killed.

A．set off

B．set out

C．set up

D．set about