Kirchhoff's Laws & Measuring Instruments question from NEET 2011 (AIPMT, Mains)

Answer: (a)

1. $V_A = 0$. From A to C the 1 V cell is crossed from − to +: $V_C = +1\text{ V}$.
2. At D: 2 A comes in from B, 1 A leaves through $R_1$, so 1 A flows D to C through $2\ \Omega$.
3. $V_D = V_C + 1 \times 2 = 3\text{ V}$
4. From D to B the 2 V cell is crossed from + to −: $V_B = 3 - 2 = +1\text{ V}$
5. So the answer is (a) +1 V.