Gaokao Math Practice Question
4. (5 分) 已知椭圆 $C: \frac{x^{2}}{a^{2}}+\frac{y^{2}}{4}=1$ 的一个焦点为 $(2,0)$, 则 $C$ 的离心率为 ( )
- A.$\frac{1}{3}$
- B.$\frac{1}{2}$
- C.$\frac{\sqrt{2}}{2}$
- D.$\frac{2 \sqrt{2}}{3}$
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