Infinite Planes Eletric Field Question

[CH1991]

Hi! I have already write everything on the pictures(In attachment), I have already circled the correct answer, however, I am not sure explain it correctly. So, can you check my explanation or question?
Thanks!

 

[Spinnor]

This may not help, but see figure 9 of the following,

http://phy214uhart.wikispaces.com/Gauss'+Law

If you take the configuration of figure 9 and copy it, paste it, flip it and connect the two copies together you get the same set up as your problem, the fields will be the same as your problem, fields zero outside and only nonzero between conducting planes.