下列计算正确的是[ ]
A.2（x+y）=2x+y
B.x4·x3=x7
C.x3-x2=x
D.（x3）2=x5
答案：B
The chair looks very old， but in fact it is very comfortable to _____.[ ]
A. sit
B. sit on
C. be sat
D. be sat on
欲建一个圆柱形无盖的净水池，要求它的容积为1000πm3，问如何选择它的直径和高，才能使所用的材料最省，最省为多少？
