Let's analyze each sentence for reliability:
A. The first pioneers to explore the opal fields found an unpleasant and unfriendly environment.
- This sentence is ambiguous. It could be based on personal experiences or subjective opinions, making it potentially unreliable.
B. None of the pioneers liked summer; it was dreadfully hot.
- This is also subjective and generalized. It cannot be considered reliable as it assumes all pioneers shared the same opinion.
C. To adapt to the challenging environment, pioneers built underground homes.
- This statement seems factual and objective, making it reliable as it likely points to a clear historical fact.
Evaluating these sentences, we can determine that:
- A is unreliable,
- B is unreliable,
- C is reliable.
Therefore, the correct answer is: A is unreliable, B is unreliable, C is reliable.