Only when class began _______that he had left his book at home.

A．will he realize

B．he did realize

C．did he realize

D．should he realize

海关为确定进出口货物的完税价格需要推迟做出估价决定时，可实行担保放行制度，但应自具保之日起( )天内核查完毕。

A．30天

B．60天

C．90天

D．180天