如欲进行“流感在某地区人群中过去的感染情况”的血清流行病学研究，下列哪种人群最适宜作为研究对象

A．自然人群

B．易感人群

C．健康人群

D．诊断明确的病例

E．现正在患流感的人群

When a bridge transmits an Ethernet frame, the Ethernet frame has （）.

A.the broadcast address for the its source address

B.the bridge's LAN address for its destination address

C.the bridge's LAN address for its source address

D.none of the above