Mum, could you buy a doll_______me? ( )
A:to
B:for
C:at
若关于x的分式方程 x−6x−5 +1= k5−x 有增根,则k的值是____.
控制细胞的活动,并且决定细胞和整个生物体遗传特性的物质是( )