$A$ galvanometer has resistance $G \ \Omega$ and $I_g$ is the current flowing through it which produces full-scale deflection. $S_1$ is the value of the shunt which converts it into an ammeter of range $0$ to $3I$,and $S_2$ is the shunt value which converts it into an ammeter of range $0$ to $4I$. The ratio $S_2:S_1$ is:

• A
  $\frac{4}{3}$
• B
  $\frac{3I-I_g}{4I-I_g}$
• C
  $\frac{3}{4}$
• D
  $\frac{4I-I_g}{3I-I_g}$