大多数天然气中烃类化合物的含量为()
A.10%~20%
B.20%~30%
C.30%~60%
D.60%~90%
参考答案:D
I have worked in this factory __________ I came here in 2001.
A. from B. for C. since
若要减小抽样误差,最切实可行的方法是
A.增加观察例数B.控制个体变异C.遵循随机化原则D.严格挑选观察对象E.选取有代表性的研究对象