According to the meaning of the question, the central angles of the two smooth arcs are less than 5, and the motion of the two balls on the arc is regarded as an equivalent simple pendulum, and the length of the equivalent pendulum is equal to the radius of the arc, then the motion periods of M and N balls are respectively

TM=2πR 1g，TN=2πR2g

When did the two balls first reach the O point?

tM= 14TM=π2R 1g，tN= 14TN=π2R2g，

Since r 1 < R2, TM < TN, the first intersection of two balls must be on the right side of point O. 。

So choose C.