In a maths paper there are 3 sections A, B & C. Section A is compulsory. Out of sections B & C a student has to attempt any one. Passing in the paper means passing in A & passing in B or C. The probability of the student passing in A, B & C are $p$, $q$ & $\frac{1}{2}$ respectively. The student selects randomly one of the options out of B & C. If the probability that the student is successful is $\frac{1}{2}$, then the possible choice can be :