Different grades are joined together by a A. Compound curve B. Transition curve C. Reverse curve D. Vertical curve

A compound curve is a combination of two or more simple curves that meet at a point. The curves can be of the same or different radii, and they can be in the same or opposite directions. Compound curves are often used in railway track to provide a smooth transition between two straight sections of track.

A reverse curve is a curve in which the direction of the track changes by 180 degrees. Reverse curves are often used in railway track to provide a way for trains to change direction without having to go through a switch.

A vertical curve is a curved section of railway track that connects two straight sections of track with different elevations. The purpose of a vertical curve is to provide a smooth transition for trains as they go from one elevation to another, preventing them from derailing.

In conclusion, the correct answer is: B. Transition curve.