A repairman maintains 4 machines. Each time a machine breaks, he repairs the machine. The machines are at a distance of 2 units from one another in a serial arrangement. At any time instance only one machine can break. If the repairman starts from the leftmost machine and walks towards the right to repair machines sequentially, what is the probability that the third machine he repairs will be the one to break?
a) 0.25
b) 0.20
c) 0.30
d) 0.15