单项选择题

( ) is extremely hard and brittle, in which much of the carbon in is retained in the form of iron carbide without graphite flakes.

A．A Grey cast irons
B．B White cast irons
C．C malleable irons
D．D SG graphite cast iron

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单项选择题

一位脱水病人在24小时缺水的情况下，则血浆中肾素的活性会降低。（）

A．对
B．错

不定项选择

干燥速率是指被干燥物料中液体的蒸发量随时间的变化率

A．正确
B．错误