电凝止血的原理是______________________。
参考答案：电热作用使血液凝结
一节干电池的电压为______，实验室用三节蓄电池串联成的蓄电池组的电压为______，我国照明电路的电压为______，人体安全电压为______．
You can’t shut your eyes to the fact ______more than 20% of American Families now live below the poverty line.
A．what
B．which
C．/
D．that
