Which of the following is the best to choose?

Question

1. Using namespace std
2. std::cout or std::cin
3. using std::cout or using std::cin

Answer 1

Whichever one you want. It doesn't matter. The creator of C++ uses 3., and most large-scale projects use 2., but it's just a convention.

Answer 2

All are the same but the easier way is using the case 1 because then if you do a big program will be easier to write cout or other