A Quad-core system is used the compute the cumulative sum of the elements in an 18-element vector. Assume that the time required on any core to perform a single addition is 2 nano-seconds. Taking into account only the time needed to perform the required additions and no other overhead, how long would it take to produce the cumulative sum if:
a) Only one core is used?
b) Two of the cores are used?
c) All four cores are used?