$3 \text{ m}/c = 10 \text{ ns}$ is the time that it takes a muon to go from one scintillation counter to the other one. Therefore, in order to distinguish upward going muons from downward going muons, the time resolution of the scintillation counters must be better than $10 \text{ ns}$ . Therefore, answer (B) is correct.

Solution to 1992 Problem 49