The athlete fell off his bike when he _____ down the hill. [      ]

A. cycled  

B. was cycling  

C. had cycled  

D. is cycling

现有下列算式：其中错误的有（　　）

（1）2a+3a=5a（2）2a•3a=5a²

（3）ax（﹣1﹣a²﹣x）=ax﹣a³x﹣ax²

（4）（x⁴﹣x³）x²=x⁶﹣x⁵

A．1个

B．2个

C．3个

D．4个