At $T \ K$,two liquids $A$ and $B$ form an ideal solution. The vapour pressures of pure liquids $A$ and $B$ at that temperature are $400 \ mm \ Hg$ and $600 \ mm \ Hg$ respectively. If the mole fraction of liquid $B$ is $0.3$ in the mixture,the mole fractions of $A$ and $B$ in the vapour phase respectively are

• A
  $0.391, 0.609$
• B
  $0.509, 0.491$
• C
  $0.609, 0.391$
• D
  $0.491, 0.509$