MAG焊时，熔化焊丝的热源主要是（），对其影响最大的焊接参数是（）。
参考答案：电弧热；焊接电流
The volunteers sent _______ books to a mountain village school on Children’s Day. [ ]
A. two hundreds of
B. two hundred of
C. two hundreds
D. two hundred
何谓泻下药？
