Evaluations for input

Water from a storm drain flows over an outfall onto a porous bed which absorbs the water at a uniform vertical velocity of 8 mm/s, as shown in Fig. P3.19. The system is 5 m deep into the paper. Find the length L of bed which will completely absorb the storm water. <image 1>

(A) 50 m

(B) 70 m

(C) 90 m

Answer with the option's letter from the given choices directly. No punctuation.