Three identical sound waves pass through an air column, a brass rod and an oil pipe of same length. In which of the three will it take the least time to reach the other end?

Three identical rings, each of mass M and radius R are placed in the same plane touching each other such that their centers from the vertices of an equilateral triangle. The M.I of the system about an axis passing through center of one of the rings and perpendicular to its plane is

Three or more lines which pass through same point are called_______

A sound wave travelling along an air column of a pipe gets reflected at the open end of the pipe. What is the phase difference between the incident and reflected waves at the open end?