为测定某组分的保留指数，气相色谱法一般采取的基准物是（）。
A、苯
B、正庚烷
C、正构烷烃
D、正丁烷和丁二烯
参考答案：C
The students were taking graduation photos on the sports ground, ______ school uniforms.
A．all of them wore
B．all wearing
C．all of whom wearing
D．all were wearing
维生素D缺乏性佝偻病激期主要临床表现是
A．烦躁、多汗B．肌肉松弛、乏力C．语言发育缓慢D．睡眠不实E．骨骼系统改变
