Answer: A, B, C
R: [OHX−]=0.01; S: [OHX−]=2×0.005=0.01, so both have pH 12. Q: [HX+]=0.01×0.04=4×10−4, pH =4−0.602=3.40. Equal volumes of strong acid and base of equal molarity neutralise exactly, pH 7. P (pH 2) is lower than Q (pH 3.40), so option 4 is false.