What i put in the space... _____ namespace std;

Question

c++

Answer 1

When you use this, we are instructing C++ compiler to use the standard c++ library. If you don't give this instruction, then you will have to use std::, each time you use a standard C++ function/entity.

Answer 2

using

Answer 3

using

Answer 4

using namespace std;

Answer 5

but what it does exactly?