A house is maintained at $21^{\circ} \mathrm{C}$ in winter by electric resistance heaters. If the outdoor temperature is $3^{\circ} \mathrm{C}$, the second-law efficiency of the resistance heaters is
(a) $0 \%$
(b) $4.1 \%$
(c) $6.1 \%$
(d) $8.6 \%$
(e) $16.3 \%$