Informal Argument for Volume of Sphere Practice Test

A student wants to apply Cavalieri’s principle to compare a sphere of radius $r$ with a cylinder of radius $r$ and height $2r$ minus two cones (each cone radius $r$, height $r$). Which step is essential for the argument?

A student tries to justify the sphere volume formula by comparing a sphere of radius $r$ to a cylinder of radius $r$ and height $2r$ with two congruent cones removed (each cone has base radius $r$ and height $r$). Which argument explains why the sphere has this volume?