Write an inequality to describe the region.
The solid upper hemisphere of the sphere of radius 2 centered at the origin.
$$z \le 0, x^2 + y^2 + z^2 \le 4$$
$$z \ge 0, x^2 + y^2 + z^2 \ge 4$$
$$z \ge 0, x^2 + y^2 + z^2 \le 4$$
$$z \ge 0, \sqrt{x^2 + y^2 + z^2} \le 4$$
none of these