下列物质中属于纯净物的是（ ）
A．洁净的食盐水
B．冰水混合物
C．净化后的空气
D．高锰酸钾完全分解后的剩余固体
答案：B
如果曲线y=2x2+3x-26上点M处的切线斜率为15，求点M的坐标。
—Is it interesting to play computer games?
—Yes, _____ you are interested in playing computer games, you’ll have trouble giving it up.
A．once
B．twice
C．as
D．as soon as
