An experiment consists of selecting a letter at random from the letters in the word IRRESISTIBLE and observing the outcomes. What is the appropriate sample space for this experiment?
a) {I, R, E, T}
b) {I, R, S, T}
c) {B, E, L, S, T}
d) {B, E, I, L, R, S, T}
e) {B, E, I, R, S, T}
f) None of the above.