What are the various methods by which you would split semi-variable costs in its fixed and variable elements?  (a) The classification of costs as controllable and non-controllable depends upon a point...

What are the various methods by which you would split semi-variable costs in its fixed and variable elements?

 (a) The classification of costs as controllable and non-controllable depends upon a point of reference. Explain.

 (b) Direct costs and controllable costs are not necessarily the same. Comment.

 (c) Why are sunk costs not relevant in decision-making?