How long would you check to see if an unconscious casualty is breathing normally ?

Question

$\begin{array}{1 1} (a)\; \text{No more than 10 seconds} \\ (b)\; \text{Approximately 10 seconds} \\ (c)\; \text{Exactly 10 seconds} \\ (d)\; \text{At least 10 seconds } \end{array}$