Answer: equal to; at their minimum.
Explanation: Marginal cost is equal to the average variable cost and the average total cost when they are at their minimum.
Thus, when average total cost is increasing, marginal cost must be above average total cost; and when at its minimum, marginal cost is equal to average total cost. Also, when average variable cost is at its minimum, marginal cost equals average variable cost.
Marginal cost is the increase in the cost that accompanies a unit increase in output; the partial derivative of the cost function with respect to output.