In which situation can fans be placed in front of a mine opening?

Fans are strategically placed at mine openings to control airflow and ensure the ventilation system is effective. Option B is particularly correct because positioning fans away from direct lines with potential forces (such as wind, dust, or hazardous gases) can prevent these elements from interfering with the airflow, thus maintaining a safe environment. This is crucial for the respiratory safety of mine workers and the overall effectiveness of the ventilation system.

In scenarios where an opening is in direct alignment with potential forces, fans could actually work against the intended airflow, leading to poor ventilation conditions. Similarly, while having no other openings may suggest a single airflow path that could benefit from fan assistance, it doesn't guarantee safety or efficacy regarding the aforementioned potential forces. The presence of mine workers is critical, but it should not solely dictate fan placement; priority must always be given to the effectiveness of ventilation systems and potential risks. Lastly, the number of fans (as suggested in the mention of having only one fan) does not inherently determine whether a fan should be placed in front of a mine opening, as multiple fans may also be required for effective airflow management, depending on the conditions and layout of the mine.