cn. a business relation in which two parties compete to gain customers
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pn. any piece of work that is undertaken or attempted
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°
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