Scientist: There is little doubt that the ice ages were caused by the unusually rich growth of vegetation worldwide.
Conclusion
The ice ages were caused by the usually rich growth of vegetation worldwide.
Evidence
Vegetation converts CO2 into O2, so a rich growth of vegetation would deplete the CO2 in the atmosphere. CO2 helps to retain the sun's heat, so if the CO2 is depleted, then the earth cools significantly, thereby causing an ice age.
Evaluate
Whenever we do a Main Conclusion or Role question, we prime our brains before reading the paragraph with the crucial reminder that the conclusion is usually earlier than the evidence!
That's the opposite of arguments in all the other question types, where the conclusion is usually the final claim (or at least in the final sentence). They are trying to hide the conclusion from us on Main Conclusion and Role, so we remind ourselves of the two big patterns: - conclusion is the 1st sentence - conclusion is some sort of "I disagree" rebuttal
When we read the first sentence, if it sounds like the author's opinion, and then the rest of the paragraph goes on to support that opinion, then we know the first claim was the Main Conclusion.
In this case, the first sentence doesn't sound quite like an opinion, because "there is little doubt" sounds like the author is reporting an established fact. But "there is little doubt" still means "we're pretty darn sure", not "we're sure". Also, the rest of the paragraph is just as scientific and factual, so we still need to pick whichever claim sits at the top of the hierarchy of ideas.
The first sentence gives us the causal chain that all the other ideas fit into.
CONCLUSION rich growth ---------------------------------> ice of vegetation ages
EVIDENCE rich growth --> CO2 to --> less CO2 ----> ice of vegetation O2 retaining heat age
Goal
Let's look for an answer that matches the meaning of the first sentence.