The cost of airing x television commercials in hundreds of d

The cost of airing x television commercials in hundreds of dollars is C(x) = 200 + 5000x - 0.4x².

What is exact cost of airing the 8th commercial in dollars?

Which is the best explanation of the following where R(x) is the revenue in dollars and P(x) is the profit in dollars and x is the item sold.

R’(6) = 35 and P’(6) = 2, the revenue from the sale of the 7th item would be roughly $35 while the profit would increase by $2.

R’(6) = 35 and P’(6) = 2, the revenue from the sale of the 6th item would be roughly $35 while the profit would increase by $2.

R’(6) = 35 and P’(6) = 2, the revenue from the sale of the 35th item would be roughly $7 while the profit would increase by $6.

R’(6) = 35 and P’(6) = 2, the revenue from the sale of the 6th item would be roughly $2 while the profit would increase by $35.

		R’(6) = 35 and P’(6) = 2, the revenue from the sale of the 7th item would be roughly $35 while the profit would increase by $2.
		R’(6) = 35 and P’(6) = 2, the revenue from the sale of the 6th item would be roughly $35 while the profit would increase by $2.
		R’(6) = 35 and P’(6) = 2, the revenue from the sale of the 35th item would be roughly $7 while the profit would increase by $6.
		R’(6) = 35 and P’(6) = 2, the revenue from the sale of the 6th item would be roughly $2 while the profit would increase by $35.

here cost of airing the 8th commercial in dollars =(C(8)-C(7))*100=(200+500*8-0.4*(8)²-200-500*7+0.4*7²)*100

=$49400

R’(6) = 35 and P’(6) = 2, the revenue from the sale of the 6th item would be roughly $35 while the profit would increase by $2