A cell with internal resistance is connected to switch $S$ and a variable resistor. A voltmeter is placed across the terminals of the cell, as shown. With switch $S$ open, the voltmeter gives a reading of $1.5\,\text{V}$. The switch is then closed and the variable resistor is set to a resistance of $4.0\,\Omega$. The voltmeter now shows $0.75\,\text{V}$. What is the internal resistance of the cell?
- A$1.0\,\Omega$
- B$2.0\,\Omega$
- C$4.0\,\Omega$
- D$8.0\,\Omega$