S2 문장의 [______]에 들어갈 단어는?
난이도 고난도 · 힌트를 덜 열수록 높은 점수 (현재 5점)
Sylvan Goldman invented the shopping cart and introduced it in his stores in 1937.
It was an excellent [______] that would make it easy for shoppers to buy as much as they wanted without getting tired or seeking others' help.