单项选择题
A.1/4 B.1/3 C.1/2 D.2/3 E.3/4
A.1/4~1/3 B.1/3~1/2 C.1/2~2/3 D.2/3~3/4 E.3/4以上
A.5° B.10° C.15° D.20° E.25°