静脉注射吸毒是艾滋病毒感染的（）
A.充分病因
B.间接病因
C.同机制有关的近因
D.必要病因
E.直接病因
参考答案：B
产品的质量特性可分为______和______。
A．物理性能
B．化学性能
C．计量值
D．计数值
Choosing to visit the Shanghai Expo on a late afternoon can make you see two to three times as many of the most popular pavilions between 6 pm and closing time ____ you hurry.
A．when
B．unless
C．where
D．if
