To assess the reliability of the sentences, we need to consider the evidence and context surrounding them.
A. The statement about the pioneers finding an "unpleasant and unfriendly environment" is subjective and lacks specific evidence or context, making it somewhat unreliable.
B. The claim that "none of the pioneers liked summer because it was dreadfully hot" is also subjective and does not provide evidence or broad consensus among pioneers, which raises its reliability.
C. The statement that "to adapt to the challenging environment, pioneers built underground homes" is a factual assertion and can be verifiable, making it more likely reliable.
Considering the analysis:
A is unreliable, B is unreliable, C is reliable.
So, the correct response would be:
A is unreliable, B is unreliable, C is reliable.