In modern times, many youths like to________others on the Internet.
A:trade in
B:trade with
C:exchange
D:exchange with
不是每一朵花都能结出果实,能结出果实的花一定( )
①长有雌蕊和雄蕊 ②长有雌蕊 ③经历了自花传粉 ④经历了传粉 ⑤经历了受精.
列竖式计算。
①67×21= ②34×84= ③85×20=
④77×23= ⑤45×30= ⑥46×33=
设Sn是等差数列{an}的前n项和,若 S3S6 = 13 ,则 S6S12 =( )