Answer
$ a.\quad$ Solid shell enclosed by the concentric spheres of radii 1 and 2, centered at the origin (the spheres are included).
$ b.\quad$ The upper half of the solid ball of radius 1, centered at the origin.
Work Step by Step
$x^{2}+y^{2}+z^{2}=1$ is a sphere of radius 1, centered at the origin.
$x^{2}+y^{2}+z^{2}=4$ is a sphere of radius $2$, centered at the origin.
$ a.\quad$
These are points between the concentric spheres of radii 1 and 2, centered at the origin (the spheres are included).
$ b.\quad$
This describes points within the sphere of radius 1, centered at the origin, above or on the xy-plane.
So, we have the upper half of the solid ball of radius 1, centered at the origin.
