first you have to find the pOH which is 9.1. Than you have to realize the number will fall between 1x 10^-9 and 1x 10^-10. the number in this case is 7.9x10^-10 which is closer to the 1x10^-9. if you think about it like i do, if the 7.9 goes to 10, that would make it 10x10^-10 which is the same thing as 1x10^-9, thus the answer being what was said because its the closest to 9.1
