--My computer ____start, no matter how hard I try.

--It might have crashed.

A．won't

B．shouldn't

C．mustn't

D．wouldn't

关系代数的运算可分为两类：传统的集合运算和专门的关系运算。下面列出的操作符中，哪些是属于传统的集合运算
Ⅰ．∪(并) Ⅱ．-(差) Ⅲ．∩(交)
Ⅳ．×(广义笛卡尔积) Ⅴ．σ(选择)
Ⅵ．为∏(投影) Ⅶ．

(连接)