The position of a mass tied to a spring hanging from the ceiling is given by x (t) = 3e-2t cost, in which positive values of x represent positions below the equilibrium position (x = 0). Assume the mass begins moving at time t = 0. When does the mass reach its highest point? O

a. at t = Arctan (-2) + pi
b. at t = Arctan (2)
c. at = Arctan ( -1/2 ) +pi
d. at t = Arctan (-2) O at t = 7