The diagram illustrates an a.c. generator. At the instant the coil moves through the position shown, the output voltage is $+10\,\text{V}$. At what point does the output voltage change to $-10\,\text{V}$?
- Awhen the coil has turned through $90^\circ$
- Bwhen the coil has turned through $180^\circ$
- Cwhen the coil has turned through $270^\circ$
- Dwhen the coil has turned through $360^\circ$