用科学记数法表示国家统计局、国务院第五次人口普查办公室公布我国人口达12.9533亿

    More people like Living in a city now, especially young people who like new and modern things. Most people think it is much ____ （容易） to find work in a city . They think there are a lot of ____ (有乐趣的) things to do and places to see in the city . There are also many ____ (选择) of public transport. When they feel tired, they can eat in good restaurants, visit museums, go to cinemas and go to parks.

    ____ （然而）, in my opinion, living in the city will ____ (花费) more . What's more, the ____ (人口) of the city is getting bigger and bigger.  so it's a ____（遗憾） that the city is always ____ （拥挤的） noisy and dirty. And It is very difficult to find a good place where people can enjoy ____（和平） and fresh air. So We should ____ （改善） the serious situation.

若式子$\sigma \left(a,b,c\right)$满足$\sigma \left(a,b,c\right)=\sigma \left(b,c,a\right)=\sigma \left(c,a,b\right)$，则称$\sigma \left(a,b,c\right)$为轮换对称式．给出如下三个式子：
①$\sigma \left(a,b,c\right)=abc$；    ②$\sigma \left(a,b,c\right)=a^2-b^2+c^2$；
③$\sigma \left(A,B,C\right)=\cos C·\cos \left(A-B\right)-{\cos }^{2}C$$(A,B,C$是$△ABC$的内角)．
其中，为轮换对称式的个数是（ ）