The test voltage and minimum insulation resistance test values for an installation forming part of a $230$ V single phase, TN$-$C$-$s system are...a$)250$ V and $1\mathrm{.}0$ MQb$)500$ V and $0\mathrm{.}5$ MQc$)500$ V and $1\mathrm{.}0$ MQd$)250$ V and $0\mathrm{.}5$ MQ