A selective precipitation question

I'm having trouble understanding the concept of selective precipitation.

Some kidney stones form by the precipitation of calcium oxalate monohydrate (CaC2O4H2O, Ksp 5 2.310^-9 ). The pH of urine varies from 5.5 to 7.0, and the average [Ca 2+] in urine is 2.6*10^-3 M.

(a) If the [oxalic acid] in urine is 3.0*10^-13 M, will kidney stones form at pH=5.5? (b) At pH = 7.0? (c) Vegetarians have a urine pH above 7. Are they more or less likely to form kidney stones?