血红素中所含有的金属离子是()
A.Fe2+
B.Ag+
C.Zn2+
D.Mg2+
E.Cu2+
参考答案:A
设点P到点(-1,0)、(1,0)距离之差为2m,到x、y轴的距离之比为2,求m的取值范围.
—How long has this bookstore been in business?
—_______ 1996.
A.After
B.In
C.From
D.Since