What is a tangent line in relation to a circle?

A tangent to a circle is a straight line which touches the circle at only one point.

Step-by-step explanation:

This point is called the point of tangency. The tangent to a circle is perpendicular to the radius at the point of tangency.

ca = ec

cpctc makes it so that corresponding parts of the triangle you proved to be equal are equal, the other options aren't part of the part of the triangles you proved to be equal.