Please don't assume things by yourself. That's the worst thing one can do on a GMAT. What you see can be misleading. Don't let it.
a. Y=2x => Nothing. Because D can be any where within the triangle. One such point is where you said - circle scenario. Remember in circle scenario the radius are equal. Here, you cannot assume that. Hence, Insuff.
b. angle DCA = 30. Again here, since we don't know AD=DC, or if DC is angular bisector. Therefore, insuff.
c. Together, still insuff because we still don't know value of angle DAC.
You haven't failed, if you haven't given up!
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