熔化极气体保护焊时，亚射流过渡采用恒流电源，依靠焊丝熔化系数的变化，改变焊丝的熔化速度。
参考答案：对
The doctor advised Vera strongly that she should take a holiday, but _____ didn't help. [ ]
A. it
B. she
C. which
D. he
月经先期的病因病机主要是：（）
A.脾虚和肾虚
B.实热和虚热
C.血瘀和血热
D.气虚和血瘀
E.气虚和血热
