I have the same question about "C". I think C does not specify the number (maybe some/most commuter will buy 5 years token, but that is just in our mind, you can not get it from stated information of this question), you can not tell how much C affect the plan, but D gave out the number "20%". Both C and D weaken the effectiveness of the plan, but D is better. Just as the OG's explanation: B and C would not have a significant impact.