Avirup-Croocked Scientist

Hello friends, I would appreciate your thankful help in solving this problem on gravitation from an IIT practice paper:- A tunnel is dug in the earth across one of its diameter. Two masses m and 2m are dropped from the ends of the tunnel. The masses collide and stick to each other and perform SHM. Then amplitude of SHM will be (when R=radius of the earth) (a) R (b) R/2 © R/root3 (d) 2R/3. I will be pleased if anyone writes the concept used and explains the steps briefly........ a fresher to the `SFN` looking forward to a reply soon. Thanking You