Suppose that in a Borda count election, outcome X is preferred to outcome Y, and outcome Y is preferred to outcome Z, when outcomes X, Y, and Z are all available options. When Y is removed as an option, however, outcome Z is preferred to outcome X. This would violate Arrow's assumption that voting systems should satisfy

a. unanimity.
b. transitivity.
c. the independence of irrelevant alternatives.
d. no dictators.

c