A solid sphere consists of a metal core, a non-conducting middle shell, and a metal outer shell. If a positive charge is placed on the core, where in the sphere will a net positive charge be found? ( )
选项：

A:evenly distributed throughout the two metal regions
B:on the outer surface of the core
C:on the outer surface of the outer shell
D:evenly distributed throughout the core
E:on both the outer surface of the core and the outer surface of the outer shell