Galileo initially assumed that the speed of a free-falling body increased uniformly with time, but at that time, it was impossible to directly measure the instantaneous speed of the object, so it was impossible to directly draw the law of the change of speed with time. Galileo concluded through mathematical operations that if the initial velocity of an object is zero and the velocity changes uniformly with time, then the displacement it passes through is proportional to the square of the time it takes. In this way, as long as we measure the time it takes for an object to pass through different displacements, we can test whether the velocity of the object changes uniformly with time. But the object falls quickly, and at this time it can only be timed by dripping water. Such a timing tool cannot measure the short time taken by a free fall. Galileo used an ingenious method to "dilute" gravity. He let a copper ball roll down the inclined plane with little resistance. The acceleration of the second ball moving on the inclined plane is much smaller than that of its vertical falling, so the time is much smaller.

So choose B.