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