Even if a firm closes down, it will still incur some fixed costs. Fixed costs are expenses that must be paid regardless of the level of production or whether the business is operating or not. These costs include things like rent, insurance, salaries of employees on long-term contracts, and equipment leases.
Variable costs, on the other hand, are expenses that vary depending on the level of production or business activity. For example, the cost of materials or labor directly related to producing goods or services. If the firm closes down, it will not incur variable costs because it will not be producing any goods or services.
Total cost is the sum of fixed and variable costs, so if the firm is not producing anything, the total cost will be equal to the fixed cost.
Marginal cost is the cost of producing one additional unit of a good or service. If the firm is closed down, there is no production, so there is no marginal cost.