The main ventilation fan fails unexpectedly in a deep$-$level underground mine. a$)$ Describe what is likely to happen to airflow within the first $15$ minutes. b$)$ Identify two areas in the mine that would be most at risk due to this failure. $(10)(2)(2)$ c$)$ Outline a three$-$step emergency response plan to restore ventilation and protect workers. $(3)$ d$)$ Recommend two long$-$term strategies to reduce the risk of total ventilation failure in future.