금성 권 고1 영어교과서 10과에
this problem was not solved until the 1930s, when the soft nylon toothbrush was invented in the us.
이거 1930년에는 문제가 해결된 건가요??정확한 해석부탁드립니당
그리고

they were small branches with a soft end that were rubbed on the teeth. 에서
that이 가르치는 것이 뭔가요? 주격관계대명사 that 맞죠??

정확한 답변 부탁드립니다^-^