What cosine function represents an amplitude of 2, a period of 2π, a horizontal shift of π, and a vertical shift of −1? (6 points) a f(x) = −1 cos πx + 2 b f(x) = −1 cos (x − π) + 2 c f(x) = 2 cos (x − π) − 1 d f(x) = 2 cos πx − 1