Mavins is a profit maximizing firm in a perfect competitive industry. Why would Mavins vary the quantity of labor units

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Mavins is a profit maximizing firm in a perfect competitive
industry. Why would Mavins vary the quantity of labor units it
purchases?
a.
Because the quantity of labor is the only input the firm can
vary
b.
To produce the profit maximizing output at the highest possible
marginal cost
c.
To produce the profit maximizing output at the lowest possible
average cost
d.
To produce the highest profit maximizing output at the lowest
possible marginal cost