21. A marble of radius r rolls without slipping down an incline and then up along a vertical circular track of radius R, as shown in the figure. What is the minimum height H from which the ball must start so that it barely stays in contact at the top of the circle? Assume r ＜＜ H and r ＜＜ R.(A)2 R(B)2.3 R(C)2.7 R(D)3 R(E)None of the above