－ How often do you have PE classes?

－ ________, Monday, Wednesday and Friday.

[      ]

A. Every three days

B. Every other day

C. Every two days

D. Every day

下列关于关系运算的叙述中正确的是

A．投影、选择、连接是从二维表的行的方向来进行运算

B．投影、选择、连接是从二维表的列的方向来进行运算

C．并、交、差是从二维表的列的方向来进行运算

D．以上三种说法都不对