Why do economists tend to oppose policies that restrict trade among nations

Dairies make low-fat milk from full-cream milk, and in the process they produce cream, which is made into ice cream. The following events occur one at a time:

(i)The wage rate of dairy workers rises.

(ii)The price of cream rises.

(iii) The price of low-fat milk rises.

(iv) With a drought forecasted, dairies raise their expected price of low-fat milk next year.

(v)New technology lowers the cost of producing ice cream.

Explain the effect of each event on the supply of low-fat milk