A 6 litre sol is 35% alcohol .How many litres of pure alcohol must be added to produce a sol that is 50% alcohol ?  Choose the correct choice from the following choices:
A) a . 0.6
B) b . 1
C) c . 2.1
D) d . 1.8
E) e . 5.4

Rationale: "35 % of 6 = 2.1 50 % of 6 = 3 shortage is 0.9 so we need to have 0.9 / 50 % to get 50 % alcohol content . = 1.8 d"
Answer: D