A、10. B、4. C、3. D、1. C四个选择是数字，初步猜测题目问的可能与数字细节有关。短文说，Johnson全家用了三年的时间攒够了里程数才换了免费机票，故答案是C。

When Johnson worked in a high-travel job more than a decade ago, he signed up for every free mileage program available. He was fortunate enough to have 100,000 miles with one airline to get four free tickets. But it took his family of four about three years to accumulate that much mileage!
    As a result, he paid little attention to the low-mileage balances in most of his frequent flier accounts. However, after he heard that the United Airline would start placing limits on people who didn’t use frequent flier miles, he checked to see whether he had any mileage due to expire at the end of the year.
    Indeed, he had a grand total of 2,805 miles, far from the 25,000 miles he needed to get one free ticket. However, there were ways he could use this small number of miles to take advantage of easy freebies. He had two choices: he could either find a low-cost, low-effort way to earn additional United miles to hold onto his 2,805 miles, or he could find some low-mileage redemption opportunities to get valuable freebies with his mileage.
    Finally he decided to[15]use 2,000 miles to get the new $100 Restaurant.com certificate with which he could get more than one dinner by using two $50 certificates. He also used his Mileage Plus number to earn 500 United miles with a participating car rental partner for a reservation he already booked.
    That kept his account active and prevented him from losing the remaining miles. It also brought his total mileage up to the mileage, which was enough to get another freebie later!
12. How many years did Johnson wait to get free tickets?
13. What did Johnson do after he heard of United Airline’s announcement?
14. How did Johnson plan to use his low-mileage balance?
15. How much mileage was left in Johnson’s account finally?