In the context of conic sections, a “completely flat ellipse” corresponds to a degenerate case where the minor axis shrinks to 0 (so $b=0$ while $a>0$). Using the ellipse eccentricity formula $e=\frac{c}{a}$ with $c^2=a^2-b^2$, what is the eccentricity of this completely flat ellipse?

• A. $e=0$
• B. $e=2$
• C. $e=0.5$
• D. $e=1$

Answer: (D)