Why do the two cycloheptadiene isomers both show four distinct shifts in carbon-13 NMR? In the following question I have filled in the correct answer for i:



For ii I thought that should be three for Q and two for P,

For Q there are three distinct enviroments for the C atoms:


$\ce{-\mathbf{C}H2-{}}$
$\ce{-\mathbf{C}H2-C=C-{}}$
$\ce{-\mathbf{C}=C&lt;}$


and for P there are two:


$\ce{-\mathbf{C}H2-C=C-{}}$
$\ce{-\mathbf{C}=C&lt;}$


Howevere the correct answer is 4 for both P and Q.

So then I thought maybe they want me to include the splitting of the peaks but in both P and Q there is too much splitting for this to be the case...

Why is the correct answer 4?
There are indeed 4 carbon environments in both of these molecules, shown below;

A plane of symmetry can be drawn through each, with one environment having only one carbon nucleus. All other sites have 2 carbons.

I don't know that any words will make it any clearer than the picture, but if you aren't sure after looking at this, I can elaborate.