The best and most correct answer among the choices provided by your question is the first choice or letter A. The swamp forests formed into tropical rain forests.
The Carboniferous Period<span> is famous for its vast swamp forests, such as the one depicted here. Such swamps produced the coal from which the term </span>Carboniferous<span>, or "carbon-bearing," is derived. The </span>Carboniferous Period<span> lasted from about 359.2 to 299 million years ago* during the late Paleozoic </span>Era<span>.</span>
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World War One, World War Two, Woodrow Wilson, Franklin D. ... focuses on the comparison of events that led to a similar situation in WWI and WWII that ... of Europe declared war against each other.
I would go with the first one
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 B is because it wasn't the polling that contributed to the act, but what the polls showed which is C.