A parallel plate capacitor with air between the plates has a capacitance of $9 \mathrm{pF}$. The separation between the plates is $d$. The space between the plates is now filled with two dielectrics constant $K_1=3$ and thickness $\frac{d}{3}$ while the other one has dielectric constant $K_2=6$ and thickness $\frac{2d}{3}$. Capacitance of the capacitor is now 

• A

  $1.8 \mathrm{pF}$
• B

  $45 \mathrm{pF}$
• Correct answer
  C

  $40.5 \mathrm{pF}$
• D

  $20.25 \mathrm{pF}$

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