At 25 °C, a solution has [H3O+]=1.0×10−3M. Which of the following best describes how its hydroxide-ion concentration compares with that of pure water at the same temperature?
a) It is higher by a factor of 104.
b) It is the same as in pure water.
c) It is lower by a factor of 104.
d) It is lower by a factor of 103.