What is a $2-$year stack$?$

a$.$ EPS forecasted two years in the future

b$.$ Percentage change in an operating metric $($revenue$,$ same store sales, etc$)$ compared to the same period two years ago

c$.$ Total return $($including dividends$)$ of a stock over the last two years

d$.$ The difference in operating margins expressed in basis points $($bps$)$ compared to the same metric two years ago