The eighth term in this series 2,6,18,54, is
1).4122
2).4374
3).5643
4).7434

by definition, an arc subtended by a central angle of some given measure has the same measure. so .

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.