已知圆O的半径为a，A，B是其圆周上的两个三等分点，则·＝
( )
A．a2
B．－a2
C．a2
D．－a2
答案：B
It is my ____ time to visit the museum. But ____ of the exhibitions have been changed. [ ]
A. two; two three
B. second; two thirds
C. two; two thirds
D. the second; two thirds
从1998年起，我国就实行无偿献血制度。提倡多少周岁的公民自愿献血。[ ]
A．15～60
B．18～55
C．18～50
D．25～60
