Although Fleming discovered Penicillin, it was over a decade ______ someone else turned it into the great drug of the 20^th century.

A．before

B．since

C．when

D．as

防火墙的基本功能是：根据一定的安全规定，检查、过滤网络之间传送的报文分组， 以确定这些报文分组的【 】。