All employees who _____ drilling equipment must wear a safety helmet 
답은 operate 입니다 who 뒤에 동사가 와야되서 오퍼레이트인건 알겟는데 보기가 
operate
operates
to operate
is operated
네개의 보기 중에 all employees 가 3인칭단수라서 저는 operates가 될줄알앗는데 아니네요 왜 안되는지가 궁금합니다 
열심히공부항게요!!