when louis pasteur introduced the idea of germs in the 1800s, people realized that all animal-hair toothbrushes eventually build up tiny bacteria, [which / in which] in turn could cause mouth diseases.
-왜 which가 정답인가요?

washing animal-hair brushes in boiling water could leave them too [soft / softly] or destroy them entirely.
-답이 soft인 이유가 too 뒤에는 형용사가 와야하니까..가 맞나요?

this problem was not solved until the 1930s, when the soft nylon toothbrush was [inventing / invented] in the us.
-답이 invented 인 이유가 수동이니까이죠!...맞죠?