阅读短文，回答问题。

Dear Amy,

   I want to 1 your friend. Let me introduce my family to you. My father is a doctor. He is tall 2 strong. My mother is a teacher. She is think and kind. My brother Sam is 6 years old. He's short and funny. We 3 in a big city. However, we usually go 4 on the weekends. I have a small and nice room. There is a bed and a desk in my room. I like reading books. I have many books 5 my desk. Welcome to my home.

                                                                                                                                                                        Yours,

                                                                                                                                                                          Lucy

金沙江下游、川滇交界的溪洛渡水电站客服窄河谷、高水头、巨泄量的技术难题，将厂房、道路全设在开挖的巨大地下洞室内，筑坝石料则全源于从洞室挖出的玄武岩。溪洛渡水电站因遵循“质量、廉洁和可持续性”的核心原则，拿下了水电站领域的“诺贝尔奖”，开启了中国水电大坝智能时代。据此完成下列问题。

已知圆$C$的极坐标方程为$\rho =a\sin \theta$，则“$a=2$”是“圆$C$与极轴所在直线相切”的  （    ）