pn. any piece of work that is undertaken or attempted
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sn. a close observer; someone who looks at something (such as an exhibition of some kind)
dn. an instrumentality invented for a particular purpose
铅垂线与H面的夹角为()。
A. 0°
B. 30°
C. 45°
D. 90°
水平线反映实长的投影是()。
A. 三投影均反映实长
B. 水平投影反映实长
C. 正面投影反映实长
D. 侧面投影反映实长