$R \longrightarrow P$ is a first order reaction. The concentration of $R$ changed from 0.04 to $0.03 \mathrm{~mol} \mathrm{~L}^{-1}$ in 40 min. What is the average velocity of the reaction in $\mathrm{mol} \mathrm{L}^{-1}$ $\mathrm{s}^{-1} ?$

• A

  $2.5 \times 10^{-4}$
• Correct answer
  B

  $4.167 \times 10^{-6}$
• C

  $4.167 \times 10^6$
• D

  $2.5 \times 10^{-5}$

Hence, the correct answer is option: 'B'.