A:must dro
B:must have dropped
C:must be dropping
D:must have been dropped
设等差数列an的前n项和为sn,若a1=-11,a4+a6=-6,则当sn取最小值时,n等于( )
下列关于基因、DNA和染色体关系的叙述,错误的是( )
18+72÷9=____ 6×7-24=____ 8×(55-48)=____