work hard in还是work hard at?

work hard in还是work hard at? work hard at hard 是work hard at sth.还是work hard on sth. work hard on和work hard at school hard at work hard at work school I(? I work hard at school. work hard at English是什么意思 At work,I must work hard All the chilldren work hard at还是work hard on our lessons work hard on the subject,work hard at English,in your lesson work hard