单项选择题

TEXT B
When the City Free Delivery Service began in America, all the letter carriers needed were their leather satchels. But, as the service continued into the 20th century, cities, populations, and commerce exploded. And before long, carders’ mailbags were stuffed to the brim, and the carders overwhelmed by the volume. Carders needed a vehicle that could get them to their routes and carry the day’s mail, including parcel post packages, without relays back to the post office. The introduction of the Parcel Post Service in 1913 had brought millions of packages into the mail stream for the first time. After World War 1I, the amount of mail delivered to American homes each day increased so dramatically that letter carders could no longer rely solely on pouches and handcarts to carry mail on their daily rounds.
In the late 1950s, the Post Office Department began supplying carriers with three-wheeled mail vans. Known as mailsters, these tiny vans were efficient in carrying mail but uncomfortable and dangerous. Mailsters offered little protection against cold or damp weather, little traction in good weather, and often no traction in snow, and poor reliability, for they were constantly breaking down. Letter carriers were not impressed by their new vehicles, which they considered too dangerous, too flimsy, too small, too underpowered, too prone to breaking down, too impractical and too top heavy. A common complaint was that mailsters were unable to compete on the open road. They simply lacked sufficient "get up and go". This frequently resulted in back-ups, with cars lined up for blocks behind the creeping three-wheelers. The tricycle design left mailsters susceptible to tipping over if cornering over 25 mph or if caught in a high wind. One mailster was even tipped over by a large dog.
The first large-scale purchase of these vehicles for letter carriers’ use was in the 1950s. These funny-looking trucks could carry up to 500 pounds of mail. By the 1960s, mailsters comprised almost one-third of the postal service’s vehicular fleet. While the idea behind mailstets was solid, the vans were not, and from the 1950s, Jeeps increasingly became a vital part of the Postal Service fleet because they were strong and agile.
According to the passage, mailsters were likely to tip over because______.