Question

http://usatestprep.com/modules/gallery/files/6/616/616.gif

C is the center of the circle.

In order to prove that opposite angles of a quadrilateral inscribed in a circle are supplementary (in this case m∠EDG + m∠EFG = 180°), which fact is MOST important to know?
A)
m∠EFD = m∠GFD
B)
m∠ECF = m∠DCG
C)
m∠EDG = m∠EFG
D)
m∠EDG =
1
2
m∠ECG