Two machines are available to perform a task: primary and back-up. Only one machine can be used on a given day and only one machine can be repaired on a given day. The primary is used whenever possible. If the primary machine has a reliability of 95% and the back-up machine has a reliability of 90%, what is the probability that the task will be successfully completed on a randomly chosen day?
a) 0.95
b) 0.90
c) 0.855
d) 0.945