C

- Hướng dẫn giải

Ta có :

\(\begin{array}{l} \bullet P = {I^2}.(R + r)\\ = I.(I.R + I.r)\\ = I.({U_R} + {U_r})\\ \bullet I = \frac{{{U_{AB}}}}{R} = \frac{{100}}{{50}} = 2(A)\\ \Rightarrow P = {I^2}.(R + r)\\ = {2^2}(50 + 20) = 280(W)\end{array}\)