In a potentiometer arrangement,$E_1$ is the cell establishing the current in the primary circuit,$E_2$ is the cell to be measured,$AB$ is the potentiometer wire,and $G$ is a galvanometer. Which of the following are the essential conditions for a balance point to be obtained?

• A
  The $emf$ of $E_1$ must be greater than the $emf$ of $E_2$.
• B
  Either the positive terminals of both $E_1$ and $E_2$ or the negative terminals of both $E_1$ and $E_2$ must be joined to one end of the potentiometer wire.
• C
  The positive terminals of $E_1$ and $E_2$ must be joined to one end of the potentiometer wire.
• D
  Both $(A)$ and $(B)$.