c
\(Q=\frac{V^2}{R_1} \times t_1=\frac{V^2}{R_2} \times t_2=\frac{V^2}{R} \times t\)

\(\frac{1}{R}=\frac{1}{R_1}+\frac{1}{R^2} \Rightarrow \frac{Q}{V^2 t}=\frac{Q}{V^2 t_1}+\frac{Q}{V^2 t_2}\)

\(\Rightarrow \frac{1}{t}=\frac{1}{t_1}+\frac{1}{t_2} \Rightarrow t=\frac{t_1 t_2}{t_1+t_2}=\frac{10 \times 40}{10+40}\)

\(=8\; min\)