Two identical cells of emf 2 V each joined in parallel provide supply to an external circuit containing two resistors of $20 \Omega$ each joined in parallel. A very high resistance voltmeter reads the terminal voltage of the cells to be 1.6 V. The internal resistance of each cell is:

$8 \Omega$ $10 \Omega$ $2.5 \Omega$ $5 \Omega$

$5 \Omega$

The correct answer is Option (4) → $5 \Omega$