A salesperson makes a base salary of $2100 per month. Once he reaches $42000 in total sales, he earns an additional 5% commission on the amount of sales over $42000. A: function to model the salesperson's total monthly salary S(x) (in dollars) as a function of the amount of sales x. B: If the salesperson had $80000 in sales, how much was his salary for that month? C: If the salesperson's total salary for a month was $4500, how much were his total sales for that month? D: Compute S(25000) and interpret what this means in the context of the problem (in terms of the monthly salary and the sales).