foo.c:2:1: error: unknown type name 'using'
    2 | using namespace std;
      | ^~~~~
foo.c:2:17: error: expected '=', ',', ';', 'asm' or '__attribute__' before 'std'
    2 | using namespace std;
      |                 ^~~
foo.c: In function 'main':
foo.c:5:4: error: 'cout' undeclared (first use in this function)
    5 |    cout<<"A"<<endl<<"AAA"<<endl<<"AAAAA"<<endl<<"AAAAAAA"<<endl;
      |    ^~~~
foo.c:5:4: note: each undeclared identifier is reported only once for each function it appears in
foo.c:5:15: error: 'endl' undeclared (first use in this function)
    5 |    cout<<"A"<<endl<<"AAA"<<endl<<"AAAAA"<<endl<<"AAAAAAA"<<endl;
      |               ^~~~

[Hydro](https://hydro.ac)提供评测服务