【C2】A．essentialB．elementaryC．necessaryD．principal

A． essentialB． availableC． reliableD． responsible

下列哪项不属于单胺类神经递质A、5－HTB、NEC、AChD、肾上腺素E、多巴胺

饲料中真正能够被动物利用的能量是（).A.GEB.NEC.DED.ME

A healthy diet is______for maintaining good overall health.A．essentialB．potentialC．regular

【C3】A．essentialB．elementaryC．necessaryD．principal

He seemed to feel the（) of some explanation.A.essentialB.wishC.necessityD.necessary