影响输差的因素正确的是（）
A.管长
B.管直径
C.管容计算值
D.输效
参考答案：C
Sam ____ some knowledge of the computer just by watching others working on it. [ ]
A. brought up
B. looked up
C. picked up
D. set up
生产经营单位确保安全生产的要求是什么？
