A diode having potential difference 0.5 V across its junction which does not depend on current, is connected in series with resistance of 20 $\mathrm{\Omega }$ across source. If 0.1 A passes through resistance then what is the voltage of the source

(a) 1.5 V                     (b) 2.0 V
(c) 2.5 V                     (d) 5 V