Four cards lie on table. Each card has a number printed on one side and a colour on the other. The faces visible on the cards are $2,3,$ red, and blue.
Proposition: If a card has an even value on one side, then its opposite face is red.
The card which MUST be turned over to verify the above proposition are
- $2,$ red
- $2,3,$ red
- $2,$ blue
- $2,$ red, blue