To determine the reliability of each sentence, we need to consider the nature of the statements:
A. "The first pioneers to explore the opal fields found an unpleasant and unfriendly environment." - This statement is subjective and likely based on individual experiences or perspectives. Therefore, it could be considered unreliable due to its subjective nature.
B. "None of the pioneers liked summer because it was dreadfully hot." - This statement also generalizes the feelings of all pioneers, making it subjective and potentially unreliable.
C. "To adapt to the challenging environment, pioneers built underground homes." - This statement describes a concrete action taken by pioneers which can be verified by historical accounts. Therefore, it is likely reliable.
Based on this analysis, the best choice is:
A is unreliable, B is unreliable, C is reliable.