Math is dumb and i can't do it

Which of the following is most likely the next step in the series? a3z, b6y, c9x, d12w, е15v, f18u

Mrs. morton has a special reward system for her class. when all her students behave well, she rewards them by putting 3 marbles into a marble jar. when the jar has 100 or more marbles, the students have a party. right now, the the jar has 24 marbles. how could mrs. morton reward the class in order for the students to have a party?

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