血COHb浓度高于50％，属于()
A.轻度中毒
B.中度中毒
C.重度中毒
D.没有中毒
E.不确定
参考答案：C
Mike is said Chinese in a Chinese university. Do you know where he studied? [ ]
A. to have studied
B. to study
C. to be studying
D. to have been studying
以无穷远处为零电势点，下述情况中各物理量为正还是为负？
（1）负电荷周围空间的电势为_________值．
（2）正电荷位于某负荷电产生的电场内，它的电势能为________值．
（3）负电荷位于某正电荷产生的电场内，它的电势能为________值．
