Average by date

We have an assignment. It says:
MOMt =(Pt-1/Pt-12)-1.
In each month t, calculate the average return of the 8 stocks that had the highest momentum in the previous month (MOMt-1).

Therefore we have to calculate the average of the return, which can be derived (column c). But before that we have to identify the in which row/stock the highest momentum was for t-1 (column d). Is there a series of formulas to give output in a single cell?

According to me, this becomes possible from row 230 and further since 12 months have past.