When It Is Necessary for Cam Followers to Travel in a Path

When it is necessary for cam followers to travel in a path that is parallel to the axis of the cam, ____.

A) a drum cam is used
B) a cylinder with a groove machined in the surface in the shape of the cam profile is used
C) the cam displacement diagram shows two parallel lines along its entire length
D) both a and b
E) all of the above