In a classic experiment carried out by the biologist Hop
dic 20 century, 20 young rats were divided into two groups.
One group was fed on a diet of pure carbohydrate, lipid and
protein; the other was fed the same diet. but with some added
milk. The rats were weighed every day. After 18 days the diets
of the two groups were swapped over.
Explain why 20 rats were used rather than just 2