A:等级分明的礼乐制度
B:“四方”臣服中央的国家观念
C:世代公卿的选官制度
D:“封建”式的地方管理方式
The smile on the teacher's face showed she was ______ with my answer.
设等差数列an的公差d≠0,a1=4d.若ak是a1与a2k的等比中项,则k=( )