\(R_Đ=\dfrac{U^2_Đ}{P_Đ}=\dfrac{6^2}{6}=6\Omega\)
\(R_{1Đ}=R_1+R_Đ=4+6=10\Omega\)
\(R_{12Đ}=\dfrac{R_{1Đ}\cdot R_2}{R_{1Đ}+R_2}=\dfrac{10\cdot6}{10+6}=3,75\Omega\)
\(R_N=R_{12Đ}+R_3=3,75+0,25=4\Omega\)
\(I_Đ=\dfrac{P_Đ}{U_Đ}=\dfrac{6}{6}=1A\)
Chọn A.