Consider the picture below which shows a plank of wood supporting a weight which is resting, in equilibrium, on two supports as shown. The system is in a constant gravitational field which acts vertically downwards. Which of the following statements about the system is correct?

http://moodle.phys.ualberta.ca/file....um-balance.png

The centre of gravity of the weight must lie between A and B and the centre of gravity of the plank must lie between C and D.

There is insufficient information to say anything about the centre of gravity.

The combined centre of gravity of the plank and weight must lie between the points B and C.

The centre of gravity of the plank alone must lie between the points B and C.

The combined centre of gravity of the plank and weight must lie between the points B and C and the plank must have a uniform density.