An unknown resistance `R_1` is connected in series with a resistance of `10 ohm`. This combination is connected to one gap of meter bridge while a resistance `R_2` is connected in other gap. The balance point is at `50 cm`. Now, when the `10 ohm` resistance is removed balance point shifts to `40 cm`. The value of `R_1` is (in ohm)

Options

A. 20
B. 10
C. 60
D. 40