A mixture consists of two spirits A and B; A evaporates at the rate of

(1) gives us the initial ratio before evaporation happened

now in 10 mins A evaporated 20 ml and b evaporated 30 ml

if the initial ratio was 4/5 lets say the total volume was 9x and A:B was 4x:5x

Pre Evaporation
A : B ::: Total Volume
4x:5x ::: 9x
Post evaporation
A : B ::: Total Volume
4x-20:5x-30 ::: 9x-50
But we dont know the value of x hence Not sufficient

(2) gives us Post evaporation ratio
A : B ::: Total Volume
5y : 4y ::: 9y

Pre evaporation it was
A : B ::: Total Volume
5y+20 : 4y +30 ::: 9y+50

But we dont know the value of y hence Not sufficient

On combining both we can solve
4x-20/9x-50 = 4y/9y
we can solve to get the value of x

Hence Sufficient C