Water is drained from a vertical cylindrical tank by opening a valve at the base of the tank. It is known that the rate at which the water level drops is proportional to the square root of water depth `y ,` where the constant of proportionality `k >0` depends on the acceleration due to gravity and the geometry of the hole. If `t` is measured in minutes and `k=1/(15),` then the time to drain the tank if the water is 4 m deep to start with is (a) 30 min (b) 45 min (c) 60 min (d) 80 min