A Proton Moving at 6 $\degree$ Above a Horizontal Plane

A proton moving at 6.0 * 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 *10 ^{- 27} kg, q_proton = 1.6 *10 ^{- 19} C)

A) 7.8 *10 ^{- 6} s
B) 1.6 *10 ^{- 6} s
C) 7.8*10 ^{- 7} s
D) 3.9 *10 ^{- 6} s