可以病例对照研究计算出的统计项是

A．相对危险度

B．比值比

C．特异危险度

D．危险差异

E．归因危险度

Some aspects of a pilot’s job _______ be boring,and pilots often _______ work at inconvenient hours.

A．can; have to

B．may; have to

C．have to; may

D．ought to; must