I believe the answer is to make the verb tenses consistent.
:)
A. Absolutely true
The 1960's and 1970's were a time that new drugs like LSD came out and people started experimenting with sex and drugs which led to the sexual revolution.
The American attitude shifted in favor of rights for Native Americans. This is based on "Polls showed that Americans believed in rights for Native Americans". Why it's not A is because it wasn't the polling that contributed to the act, but what the polls showed which is C.
