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You buy stamps at a post office.

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The numbers of known hydrocarbons run into tens of thousands.

To our knowledge, advertisement and commercials do many important things for society.

1. 图样中的尺寸标注的单位是（）

A. m
B. cm
C. mm
D. μm

不可见轮廓线应采用（）线性来绘制。

A. 粗实线
B. 细虚线
C. 细实线
D. 细点划线