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Let $U$ be the set of points in space whose cylindrical coordinates are the same as their spherical coordinates, in the same order.

Which of the following statements is TRUE?

The set $U$ consists of a plane that is not perpendicular to any one of the coordinate axes.

The set $U$ consists of one point.

The set $U$ consists of a line.

The set $U$ consists of a plane that is perpendicular to one of the coordinate axes.

The set $U$ is empty.