________다음 문장에서 밑줄친 부분이 왜 과거완료 형태로 쓰였는지 궁금합니다. 과거완료를 쓰려면 비교하는 기준이 되는 과거시제가 쓰여야 하는데 아무리 찾아도 그 문장안에 과거시제로 쓰인 부분을 찾을 수가 없네요. 고수님들의 소중한답변 부탁드립니다.

but no one knew how to build an ac motor, and tesla’s professor told him it was impossible.
tesla was not disappointed and he continued to develop his idea.

after university, tesla had left for new york to work for a company founded by thomas edison.
at that time, edison was using dc electricity,
which couldn’t travel through a wire for more than a mile.