2017 AA Question 1 - Comp Adder

We learned that a Comb Adder gets two strings with length n and outputs one string with length n+1. It seems that in the answer shown here they used the Comb Adder without taking into account the fact that it outputs an extra bit. Is this a mistake in the solution?
Thank you

I don’t know what is the question but I assume that the functionality only requires the 2n least significant bits. For every combinational component you can use only a constant part of the outputs if that is only needed for the functionality. So I assume this is not by mistake.