Identify if the three sentences are reliable or unreliable.

To evaluate the reliability of the sentences, we need to consider whether they can be substantiated by historical records or credible sources. However, based on general knowledge and the nature of the claims, we can make some assessments.

A. "The first pioneers to explore the opal fields found an unpleasant and unfriendly environment." - This sentence is somewhat subjective, as "unpleasant" and "unfriendly" can vary based on personal experience. While it's plausible that the environment could have been challenging, the wording seems to lack specificity and evidence, making it potentially unreliable.

B. "None of the pioneers liked summer because it was dreadfully hot." - This statement makes a broad claim about all pioneers and their feelings towards summer. It is unlikely that every individual had the same sentiment, which could make this statement unreliable.

C. "To adapt to the challenging environment, pioneers built underground homes." - This is a factual statement that can be verified and is consistent with historical practices of people adapting to harsh environments. Therefore, it appears to be reliable.

Based on this analysis, the most fitting choice is:

A is unreliable, B is unreliable, C is reliable.