How are federal and unitary systems of government different ?

In a unitary system, the central or national government has almost all of the power. This is different from a federal government in which power is divided by a written Constitution between a central government and other governments.

The correct answer is D.

The cost has risen 1,000$ (comparting the 1500$ he had paid, with the 2500$ his friend has offered for the vacation, 1000$ is the amount he would miss if rejecting the offer plus the value of the activity he would do instead of the skiing vacation).

This cost rise represents the concept of opportunity cost. It is referred, when making a decision, to the value that is missed when rejecting the alternative option.