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如图,点C在⊙O上,若∠ACB=40°,则∠AOB等于( )
A.40° B.60° C.80° D.100°
如图,A、B、C三点在⊙O上,若∠AOB=100°,则∠ACB等于
A.90°
B.80°
C.50°
D.40°