降低通风阻力的措施是（）。
A、采用较长的通风管
B、采用小直径通风管
C、增多和加急风管拐弯处
D、保持风管顺直和平稳
参考答案：D
His uncle ______ for more than 9 years.[ ]
A. has come here
B. has started to work
C. has lived there
D. has left the university
已知，（x+y）2=16，（x﹣y）2=8，那么xy的值是[ ]
A．﹣2
B．2
C．﹣3
D．3
