How to write row numbers linq to query in C#?

Select top percent a,b,c,d,ROW_NUMBER() over(PARTITION BY a,b order by a,b) as rowsnumber
from Students as s
group by a,b,c,d

var students=from p in Students
group p by new{p.a,p.b} into grp
order by grp.Key.a,grp.Key.b
.Select(s=>s.a,s.b,s.c,s.d).ToList();