Milk is the best food. It has water in it, sugar, fat, vitamins, and proteins. People drink milk from different animals. Cows are reared for milk in England, New Zealand, and many other cold countries. In hot, dry countries like Arabia and the middle of Asia, camels are reared for milk. In India, both buffaloes and cows are raised for milk. In many places, goats are kept for milk. The Eskimos keep herds of reindeer. They live in the very cold countries of North America. From milk, they can make butter and cheese. The milk we use must be pure and germ-free. Impure milk does more harm than good to the human body.

Choose the correct option to answer each question.

Which word in the passage is the antonym of 'secondary'?