研磨圆柱孔时，出现孔的两端大，中间小的主要原因是（）
A.研磨棒与孔间隙太大
B.研磨棒与孔间隙太小
C.磨料有问题
D.磨料要用机油调和
参考答案：B
有X、Y、Z三种金属，把它们投入稀盐酸中，只有Y表面有气泡冒出，把X投入到Z的硝酸盐溶液中，X表面有Z析出，X、Y、Z三种金属的活动性由强到弱的顺序是（ ）
A．Z＞Y＞X
B．Y＞X＞Z
C．X＞Y＞Z
D．X＞Z＞Y
All don''t have a free ticket must pay the admission fee.
A．Everyone who doesn't have a free ticket
B．No one who doesn't have a free ticket
C．No one who has free tickets
D．Anyone who has free tickets
