Chinese writing began thousands of years ago. According ____ an ancient story, a man named Cang Jie invented Chinese writing. One winter day while he was hunting, he saw the ____(track) of animals in the snow and ____(observe) that the appearance of each one was different. Then ____he got was that he could use different shapes ____(represent) different objects. The ____(one) Chinese characters were drawings of physical objects. Some characters ____(simplify) and others have been made ____(difficult) over time.____, as a whole, the characters have ____(gradual) developed from drawings into standard forms.

不为0的四个实数a、b，c、d满足ab=cd，改写成比例式错误的是（　　）