How is momentum calculated in the stock market?

A$)$

By taking the last $12$ months of returns and finding the average return of the top $50\%$ of assets minus the average return of the bottom $50\%$

B$)$

By taking the last $12$ months of returns and finding the average return of the top $30\%$ of assets minus the average return of the bottom $30\%$

C$)$

By taking the last $6$ months of returns and finding the average return of the top $30\%$ of assets minus the average return of the bottom $30\%$

D$)$

By taking the last $6$ months of returns and finding the average return of the top $50\%$ of assets minus the average return of the bottom $50\%$