在人类生长发育过程中，哪个系统是优先发育的

A．生殖系统

B．内分泌系统

C．心血管系统

D．骨骼系统

E．神经系统

The place _____ the bridge is supposed to be built should be _____ the cross-river traffic is heaviest.

A．which; where

B．at which; which

C．at which; where

D．which; in which