A potentiometer wire has length `4 m` and resistance `8 Omega`. The resistance that must be connected in series with the wire and an accumulator of e.m.f. `2 V`, so as the get a potential gradient `1 mV` per cm` on the wire is
A. `40 Omega`
B. `44 Omega`
C. `48 Omega`
D. ` 52 Omega`