7. 밑줄 친 부분 중 어법상 옳지 않은 것을 고르시오.
Domesticated animals are the earliest and most effective 'machines' ①available to humans. They take the strain off the human back and arms. ②Utilizing with other techniques, animals can raise human living standards very considerably, both as supplementary foodstuffs (protein in meat and milk) and as machines ③to carry burdens, lift water, and grind grain. Since they are so obviously ④of 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.
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