c
(c) \(\frac{H}{t} = P = \frac{{{V^2}}}{R} \Rightarrow P \propto \frac{1}{R}\) also \(R \propto \frac{l}{A} \propto \frac{{{l^2}\rho }}{{A.l\rho }}\)
\( \Rightarrow \) \(R \propto \frac{{{l^2}}}{m}\) \( \Rightarrow \) \(R \propto {l^2}\) (for same mass)
So \(\frac{{{P_A}}}{{{P_B}}} = \frac{{l_B^2}}{{l_A^2}} = \frac{4}{1} \Rightarrow {P_A} = 20\,W\)