forces - Trying to understand the tension in a spring with two weights attached

Possible Duplicate:
Why I think tension should be twice the force in a tug of war

For (A) in this image, I would think the tension should be should be 2mg.

However, my book says the weight in (A) should be only mg. I don't understand why the tension should not be the same as in (C) and double the tension in (B).

Answer

As you can see a) and c) are different because in a) the spring is part of the rope and in c) it is part of the support.

The tension in a) and b) is the same as the systems are in static equilibrium.