Fractionating columns normally work better if they are insulated in order to maintain a smooth temperature gradient in the column. Why is there a temperature gradient in a fractionating column?

A. As you move up the column, the vapour becomes more enriched in the lower boiling fraction so the boiling point of the liquid decreases.

B. As you move up the column, the vapour becomes more enriched in the higher boiling fraction so the boiling point of the liquid decreases.

C. As you move up the column, the vapour becomes more enriched in the lower boiling fraction so the boiling point of the liquid increases.

D. As you move up the column, the vapour becomes more enriched in the higher boiling fraction so the boiling point of the liquid increases.