Hi,
This type of question often trips me up especially when I am not careful!
Recall the Kw equation where Kw = [H+][OH-], where at 25 degrees C Kw = 1 x10^-14, and [H+] = 1x10^-7 M.
Here, as the concentration given of acid is 10^-8 M, which is one order of magnitude lower than the auto ionization of water where [H+] = 1x10^-7. In cases where the acid concentration is lower than 10^-7, we treat water as the main contributor for the acid dissociation.