喷油泵有哪些部件组成?
参考答案:由柱塞套筒偶件、传动零件(凸轮轴、凸轮推杆)、出油阀、油量调节机构(齿条、齿环、旋转套筒)及泵体组成。
It’s difficult for me to decide which one to choose, because ______ of them are good.
A.neither
B.both
C.either
D.each
对下丘脑和垂体有负反馈作用,抑制促性腺激素的分泌()
A.儿茶酚胺
B.β内啡肽
C.GnRH
D.孕激素
E.雌激素