One ball is dropped while another ball is thrown downward with 10 m/s simultaneously then the separation of the two bodies will be 18 m after

Question
 One ball is dropped while another ball is thrown downward with 10 m/s simultaneously then the separation of the two bodies will be 18 m after 

a. 1.8 sec

b. 9 sec
c. 4.5 sec
d. 3.6 sec