A$.$ What is stoichiometry $,$ stoichiometric relation how does it affect a reaction?$($ plz explain easy and if needed using analogy$)$

B$.$ what is meant by when Said that $"$ for every $2$ moles of hydrogen reacting $,1$ mole of oxygen is consumed

$($ Is it connected to law of conservation of mass? Of so why and what it means?$)$

C$.$What is mole $($ I know it's a quantity$)$ but of what atoms $,$ electrons particles $?$what is avagadro number and what is its connection to mole.

D$.$ What is atom balances In Numerical of chemistry, why is it needed and use and how does it affect if we don't do it$?$

E $.$how much quantity does a mole have eg$.1$ mole of hydrogen has how many atom or particles, does every element have different?