$A$ galvanometer can be converted to a voltmeter of full-scale deflection $V_{0}$ by connecting a series resistance $R_{1}$ and can be converted to an ammeter of full-scale deflection $I_{0}$ by connecting a shunt resistance $R_{2}$. What is the current flowing through the galvanometer at its full-scale deflection?

• A
  $\frac{V_{0}-I_{0} R_{2}}{R_{1}-R_{2}}$
• B
  $\frac{V_{0}+I_{0} R_{2}}{R_{1}+R_{2}}$
• C
  $\frac{V_{0}-I_{0} R_{2}}{R_{2}-R_{1}}$
• D
  $\frac{V_{0}+I_{0} R_{1}}{R_{1}+R_{2}}$