NOVA Online | Voyage of Doom | Buoyancy Question

Buoyancy Question
Question 1 sub

Here we have a toy submarine floating in a bathtub. It's a really fancy sub, made out of steel. The sub weighs one pound. When completely submerged, it displaces two pounds of water.

What could you do to cause the sub to sink to the bottom of the tub?

Add one pound of sand to the sub's interior.
Add one pound of sand to the sub's interior, plus a little more.
Nothing. Since the boat displaces more water than it weighs, it's already on its way down.