A cardboard tube has two wires, P and Q, attached to its ends. Within the tube, wire P and wire Q are connected to two resistors. The potential difference (p.d.) between wire P and wire Q is $9.0\text{ V}$ and the current in the wires is $0.75\text{ A}$. Which combination of resistors is inside the tube?
- Aa $0.75\,\Omega$ resistor and a $6.0\,\Omega$ resistor in parallel
- Ba $0.75\,\Omega$ resistor and a $6.0\,\Omega$ resistor in series
- Ca $20\,\Omega$ resistor and a $30\,\Omega$ resistor in parallel
- Da $20\,\Omega$ resistor and a $30\,\Omega$ resistor in series