A Proton Moving at 3 $\degree$ Above a Horizontal Plane

A proton moving at 3.0 x 10⁴ m/s is projected at an angle of 30 $\degree$ above a horizontal plane.If an electric field of 400 N/C is acting down,how long does it take the proton to return to the horizontal plane? (Hint: Ignore gravity.m_proton = 1.67 x 10^-27 kg,q_proton = 1.6 x 10^-19 C)

A) 7.8 x 10^-7 s
B) 1.7 x 10^-6 s
C) 3.9 x 10^-6 s
D) 7.8 x 10^-6 s