At $27^{\circ} C$,a closed vessel contains a mixture of equal weights of helium (mol. wt. = $4$),methane (mol. wt. = $16$) and sulphur dioxide (mol. wt. = $64$). The pressure exerted by the mixture is $210 \ mm$. If the partial pressures of helium,methane and sulphur dioxide are $p_1, p_2$ and $p_3$ respectively,which one of the following is correct?

• A
  $p_3 > p_2 > p_1$
• B
  $p_1 > p_2 > p_3$
• C
  $p_1 > p_3 > p_2$
• D
  $p_2 > p_3 > p_1$