The function a represents the cost of manufacturing product A, in hundreds of dollars, and the function b represents the cost of manufacturing product B, in hundreds of dollars.

a(t) = 5t + 2
b(t) = 7t - 2t +4

Find the expression that describes the total cost of manufacturing both products, a(t) + b(t).

A. 7t^2 - 7t + 2

B. 7t^2 + 3 +6

C. 7t^2 + 7t - 6

D. 7t^2 - 3t + 6