(21) A model for advertising response is given by N01) = 10000 + 80001n a where Nla) = the number of units sold and a = the amount spent on advertising, in thousands of dollars. Find the rate at which the number of units sold is changing after spending $4000 on advertising? 0 200 units per $1,000 spent on advertising 0 2000 units per $1,000 spent on advertising 0 20 units per $1,000 spent on advertising 0 2 units per $1,000 spent on advertising