her parents promised ( ) her a computer if she got good grades.

问题描述:

her parents promised ( ) her a computer if she got good grades.
her parents promised () her a computer if she got good grades.
A、buying B、to buyC、bought D、will buy

B
promise to do sth.:承诺做某事
句意:他的父母承诺当他考得好的时候,将买一台电脑给他.
多谢!if引导的这句话为什么不选A?