What is the minimum quantity of air required across the last open crosscut?

The minimum quantity of air required across the last open crosscut is determined primarily by safety standards and regulations that aim to ensure adequate ventilation in mining operations. The correct response sets the standard at 9,000 cubic feet per minute, which is designed to maintain sufficient airflow to dilute and remove potentially hazardous gases, provide oxygen for miners, and manage heat levels within the mine.

This quantity is critical in a coal mine where explosive gases such as methane can accumulate, and where the risk of asphyxiation also exists due to insufficient oxygen levels. Adequate ventilation not only helps prevent accidents and health issues but also complies with legal requirements that have been established to protect miners' health and safety.

While the other options present different volumes of air, they do not meet the current regulatory standards necessary to ensure miner safety in environments where air quality and circulation can significantly impact worker health. Therefore, the choice of 9,000 cubic feet per minute reflects an adherence to these stringent safety guidelines, establishing it as the correct answer.