My father served in the army in _____ when he was in _____. [ ]

A. 1950's; twenties

B. the 1950's; his twenties

C. the 1950's; the twenties

D. 1950's; the twenties

伊朗核问题是当今世界的热点之一。古代伊朗以波斯人为中心形成了波斯帝国。请问：波斯帝国形成于[ ]

A．公元前4世纪

B．公元前5世纪

C．公元前6世纪

D．公元前7世纪