129. On a recent trip, Cindy drove her car 290 miles, rounded to the nearest 10 miles, and used 12 gallons of gasoline, rounded to the nearest gallon. The actual number of miles per gallon that Cindy’s car got on this trip must have been between.

(D) 285/12.5 and 298/11.5

解释说， since the gallons of gasoline are rounded to the nearest gallon, they can range anywhere from 11.5 to 12.5 gallons.

为什么呢？