一个高为15cm的圆柱形笔筒，底面圆的半径为5cm，那么它的侧面积为______cm2（结果保留π）．
它的侧面积为2π×5×15=150πcm2．
.
______is known to us all, the 2008 Olympic Games took place in Beijing.
A．It
B．As
C．What
D．Which
He didn’t go to school yesterday ________ he was ill.
A．because
B．because of
C．if
D．so
