One junction of a certain thermoelectric couple is at a fixed temperature $T_r$ and the other junction is at temperature $T$. The thermo-electromotive force for this is expressed by $E=k(T-T_r)[T_0-\frac{1}{2}(T+T_r)]$. At temperature $T=\frac{1}{2} T_0$, the thermoelectric power is

• A
  $\frac{1}{2} k T_0$
• B
  $k T_0$
• C
  $\frac{1}{2} k T_0^2$
• D
  $\frac{1}{2} k(T_0-T_r)^2$