A:禅让制出现
B:世袭制出现
C:分封制出现
D:郡县制出现
There are on my father's farm, so we often have some delicious to eat.
下列有关仪器使用方法或实验操作正确的是( )
已知an为等差数列,其前n项和为Sn,若a3=6,S3=12,则公差d等于( )