If the firm closed down in the short run and produced zero units of output, its total cost would be $50 (first option).
The short run is a period when at least one or more factors of production are fixed. In the short run, if the average variable cost is greater than the price at which the firm sells its good or service, the firm should shut down.
When the firm shuts down, it would still incur its fixed cost. Fixed cost is the cost that is incurred regardless the level of output. Fixed cost is the product of output and the average fixed cost.
Fixed cost = output x average fixed cost
1 x $50 = $50
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