单项选择题

典型苯丙酮尿症的病因是由于肝脏缺乏（）

A、多巴胺
B、5-羟色胺
C、四氢生物蝶呤
D、酪氨酸羟化酶
E、苯丙氨酸羟化酶

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According to Philip Balls research, which of the following is true?
A．Human beings are born to be professional musicians.
B．There exist no sharp differences among various types of music.
C．People usually learn music by means of hearing around.
D．Mankind posses a natural music instinct and can't wipe it off.

A.Human
B.
B．There
C.
C．People
D.
D．Mankind
E.

单项选择题

数控机床中电力拖动系统的运行状态有静止、起动、稳速、加速、减速和()等运动状态。

A.悬停
B.匀加速
C.匀减速
D.制动