For a $1^{st}$ order reaction $R \rightarrow P$, the concentration of reactant $R$ changes from $0.1 \text{ M}$ to $0.025 \text{ M}$ in $40 \text{ minutes}$. The rate of reaction when the concentration of $R$ is $0.01 \text{ M}$ is

• A
  $1.73 \times 10^{-5} \text{ M min}^{-1}$
• B
  $3.47 \times 10^{-4} \text{ M min}^{-1}$
• C
  $3.47 \times 10^{-5} \text{ M min}^{-1}$
• D
  $1.73 \times 10^{-4} \text{ M min}^{-1}$