The thermo e.m of a hypothetical thermocouple varies with the temperature $\theta$ of hot junction as $E= a \theta + b \theta^2$ in volts, where the ratio $a/b$ is $700^{\circ}C$. If the cold junction is kept at $0^{\circ}C,$ then the neutral temperature is :

Question

$$\begin {array} {1 1} (a)\;700^{\circ}C \\ (b)\;1400^{\circ}C \\ (c)\;390^{\circ}C \\ (d)\;no\;neutral\;temperature \;is\;possible\;for\;this\;thermocouple;\end {array}$$