Marisol's Parasols sells novelty umbrellas for $10 each. Marisol's variable costs are $4 per unit, and her fixed expenses are $3,000 per month. If Marisol's tax rate is 25%, how many umbrellas must Marisol sell each month if she wants to earn $9,000 in net income?
a. 500
b. 2,000
c. 2,500
d. 3,500