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