There are all kinds of __________ in the shop.
A:fruit
B:fruits
C:breads
D:book
观察玉米种子结构时,在纵切面上滴一滴碘液,目的是( )
如图,点A1、A2、A3、…,点B1、B2、B3、…,分别在射线OM、ON上,A1B1∥A2B2∥A3B3∥A4B4∥….如果A1B1=2,A1A2=2OA1,A2A3=3OA1,A3A4=4OA1,….那么A2B2=____,AnBn=____.(n为正整数)