单项选择题

（）适合制机床主轴。

A.20CrMnTi
B.45
C.20
D.9SiCr

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单项选择题

According to the nature ofcause of disease, plant diseases are classified as follows:
A、Infectious diseases and Noninfectious diseases
B、Infectious diseases and biological diseases
C、Noninfectious diseases and abiotic diseases
D、Infectious diseases and biotic diseases

单项选择题

手机街拍要注意时间的选择，早晨和傍晚的光线都会创造出很多有趣的光影效果。

A．对
B．错