He's not seen his mother for a long time, __________ he?
A:does
B:has
C:doesn't
D:hasn't
一 My cousin is a clever boy and he is good at math.
一 So he is.
A、B、C是初中化学常见的物质,它们之间的转化关系如下图所示(所需反应条件已略去).据图回答问题: