在$△ABC$中，$A:B:C=1:2:3$，则$a:b:c=$（    ）

完形填空

    The 2013 Talent Show in our school was a night to remember! It was a great1Students, parents and teachers2, sang and played instruments. There were more than thirty3. Our English teacher Jason Cruise4an English song. He won the prize 5 the best performer. 6a reporter from our school magazine interviewed him, he said that singing was7important to him. The prize for the funniest act8to Paul Black, because he danced like a drunken (喝醉的) man without music. And his classmates said he was the9boy in the class.

    Many thanks to the parent and the student volunteers. They made this10show possible. Many thanks to all of the people who helped.

已知集合$M=\left\{x|\left|x-7\right|<9\right\}$，$N=\left\{x|y=\sqrt{9-x^2}\right\}$，且$M,N$都是全集$U$的子集，则右图中阴影部分表示的集合是        （   ）