—Do you have a basketball? —______.
A:Yes, I do
B:I do
C:Yes, I have
D:Yes, I've
The students their pens when the teacher told them to stop writing.
如果一个氨基酸分子中有两个羧基,其中一个羧基连接在R基上,那么另一羧基的部位是在 ( )