I made the same mistake. It was pretty subtle.
If you are to look at the fattest leg at the very back, the 3D drawing shows the left side to be thinner than the right side. B shows them as equal size.
One thing I recommend is going through the solutions at the top right of the achiever program (you can download the program for free to play the files). You can do cool stuff like rotate it and figure out why an answer is right/wrong.
