----Does Liu Hua serve in the army?

----No,but he ___in the army for three years.

A．served

B．has served

C．is serving

D．would serve

用异烟肼时合用维生素B₆的目的是

A．促进异烟肼吸收

B．增强异烟肼的疗效

C．延缓耐药性产生

D．降低异烟肼对神经的毒性

E．减轻异烟肼对肝脏的毒性