I'm going to work hard-----it

B。at
work hard at: 努力工作(或学习)
例如:
1。 If you work hard at other times, you won't have to sit up all night before the exam。
你平时如果努力学习,在考试前就不必这样整夜开夜车了。

2。 He worked hard at his biology。
他努力学习生物学。