From the top of a tower, a stone is thrown up and reaches the ground in time t₁= 9s. A second stone is thrown down with the same speed and reaches the ground in time t₂ = 4s. A third stone is released from rest and reaches the ground in time t₃, which is equal to
a. 6.5 s
b. 6 s
c. 5/36 s
d. 65 s