Horsemen agree there is no more difficult race to get ready for than the Kentucky Derby. The race comes at a time of year when most of the entries have barely turned three years old. Because the universal thoroughbred birthday is January 1, Riva Ridge, the 1972 winner, for example, was officially three on January 1, 1972. In truth, the horse was not 36 months old until April 13, a scant three weeks before the Derby. At a time when thoroughbreds are still growing fast, and their bones are still soft, they are asked for the first time to run a mile and a quarter carrying 126 pounds, or 121 pounds for fillies, against the best available competition in the nation. The field is invariably big, and a certain number of cheap horses always clutter the race.

Q. Underline a sentence which reveals the pressure on young race horses.