Standard enthalpy of vapourisation $\bigtriangleup _{vap}H^{\ominus}$ for water at 100°C is $40.66 kJ mol^{-1}$. The internal energy of vaporisation of water at 100°C (in $kJmol^{-1}$) is :
( A ) $+40.66$
( B ) $+37.56$
( C ) $+43.76$
( D ) $-43.76$