In the adjoining figure, every point of circle $O'$ is exterior to circle $O.$ Let $P$ and $Q$ be the points of intersection of an internal common tangent with the two external common tangents. Then the length of $PQ$ is
the average of the lengths of the internal and external common
tangents
equal to the length of an external common tangent if and only
if circles $O$ and $O'$ have equal radii
always equal to the length of an external
common tangent
greater than the length of an external common tangent
the geometric mean of the lengths of the internal and external
common tangents
