纯天然气的爆炸极限为（）。
A.5%～15%
B.15%～25%
C.5%～30%
D.35%～65%
参考答案：A
f（x）=ax3-3x+1对于x∈[-1，1]总有f（x）≥0成立，则a=______．
Do you think there robots in people’s homes in the future?
A. will be
B. will is
C. be will
D will in
