Which of the following best describes 'zero-based budgeting'?

a) A method of budgeting where the sum of costs and revenues for each budget centre equals zero

b) A method of budgeting that distinguishes fixed and variable cost behaviour with respect to changes in output and the budget is designed to change appropriately with such fluctuations

c) A method of budgeting whereby all activities are re-evaluated each time a budget is formulated

d) A budget method where an attempt is made to make expenditure under each cost heading as close to zero as possible