What is the MTBF of the system?

The operating time of a system is 24 hours per day, Monday through Saturday.

Failure of the system occurs as follows.

Monday: No failure

Tuesday: 5 A. M. to 7 A. M. Wednesday: No failure

Thursday: 4 P.M. to 8 P.M.

Friday: 8 A. M. to 11 A. M. Saturday: No failure

What is the MTBF of the system?

A. 3 hours

B. 8 hours

C. 45 hours

D. 56 hours