2009年入秋以来，我国云南、贵州、广西、重庆、四川等五省（区、市）遭遇历史上罕见的特大[ ]
A．沙尘暴
B．雪灾
C．旱灾
D．洪灾
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
电焊机把线必须到位，地线可借用结构钢筋，但要注意防止火灾。
