[9급 국가직 영어 2019년 7번]

Domesticated animals are the earliest and most effective 'machines' ① available to humans. They take the strain off the human back and arms. ② Utilizing with other techniques, animals can raise human living standards very considerably, both as supplementary foodstuffs (protein in meat and milk) and as machines ③ to carry burdens, lift water, and grind grain. Since they are so obviously ④ of great benefit, we might expect to find that over the centuries humans would increase the number and quality of the animals they kept. Surprisingly, this has not usually been the case.