Which of the following statements is true?
a. it is usually less risky to use single suppliers rather than multiple suppliers.
b. when multiple suppliers are used, having geographically centralized suppliers could lessen the probability of all failing simultaneously.
c. when multiple suppliers are used, having geographically dispersed suppliers could lessen the probability of all failing simultaneously.
d. the risk mitigation effectiveness of using multiple suppliers has nothing to do with the probability of a super-event occurring.