A ball is thrown vertically upwards with speed 7.7ms^-1 from the top of a sheer cliff of height 21m.The time taken by the ball to reach the foot of the cliff is-[Assume g = 9.8ms^-2]

A \( \frac{10}{7} s \)

B \( \frac{11}{7} s \)

C \( 5 s \)