Trains are operated in both directions on a single track in under river tunnels whenever certain kinds of work must be done, alternate trains running in opposite directions. If the running time between the two towers at the ends of a certain tunnel is 41/2 minutes, the MINIMUM headway in one direction when such single-tracking is operated is nearest to____minutes:
a. 10
b. 5
c. 4.5
d. 2.25