单选题 《复合题被拆开情况》 1. Book and pay for your car rental in advance Discounts are available from many car rental companies. In most cases, discounts are given to those who prepay for their rental at the time of reservation (预订) rather than when they return the vehicle. 2. Use discount or coupon codes (优惠码) Some car rental companies offer special coupon codes to help save on various rentals and sometimes include one-way rentals. Coupon codes can usually be found on car rental company websites. 3. Follow the seasons During certain times of the year, car rental companies need to transport their fleet (车队) from one part of the country to another. Car rental companies tend to move their fleets during the winter months and spring. During these times, you are headed in the right direction, you may get better rates in one-way car rental. 4. Insurance By taking rental car insurance through a car rental company, you can save money in the long run, even if you have insurance.《问题》：What do car rental companies usually do during the winter months and spring?