肾性骨病早期诊断最可靠的依据（）.
A.血PTH浓度测定
B.症状表现
C.骨活检
D.血钙测定
E.骨X线片
参考答案：C
— It’s already Monday. Sam lost his MP4 last Wednesday and he hasn’t found it.
— Really? Do you mean_?
A．did Sam lose his MP4 five days ago
B．Sam lost his MP4 for five days
C．it’s five days since Sam lost his MP4
D．has Sam lost his MP4 for five days
以总椭球为根据的坐标系，叫做（）坐标系，这是一个（）直角坐标系。
