In an ER diagram, what would be the foreign keys be in the following entitys?

INVOICE $($INV$\_$NUM $($Primary Key$),$ PROD$\_$NUM, SALE$\_$DATE, PROD$\_$LABEL$)$

VENDOR$($VEND$\_$CODE $($Primary Key$),$ VEND$\_$NAME, QUANT$\_$SOLD, PROD$\_$PRICE$)$