electric fields?

Question

Which diagram represents the net electric field between two charged parallel plates if a neutral conducting sphere is placed between the plates? (answers in the book say none of them)

http://img517.imageshack.us/img517/3172/img0436yq9.jpg

would appreciate a detailed explanation.

Answer 1

Note that in general the lines should be going basically from positive to negative, so (c) is out.

(a) and (d) both have all the lines touching the sphere connected to the same type of charge. This is only possible if the sphere is charged (positively for (a), negatively for (d)). Since the sphere is neutral, these answers are incorrect.

(b) implies that there is a potential difference between the top and bottom of the sphere. Since it is conducting, this is incorrect. The true picture would look similar to (b), but with lines detouring around the sphere.