通风系统按进风与出风井的位置可分为（）。
A．中央式、两翼式
B．压入式、抽出式
C．压入式、混合式
参考答案：A
-（-2）+4=______．
—Nowadays the doctors are trying their best to reduce the people’s fear__________ they would be affected by the present disease called “Bird Flu （禽流感）”.
A．whether
B．which
C．that
D．when
