Bài làm

R₁ và R₂ mắc nối tiếp nên ${{\text{R}}_{\text{12}}}\text{=}{{\text{R}}_{\text{1}}}\text{+}{{\text{R}}_{\text{2}}}\Leftrightarrow \text{12=2}{{\text{R}}_{\text{2}}}\text{+}{{\text{R}}_{\text{2}}}\Leftrightarrow {{\text{R}}_{\text{2}}}\text{=4 }\!\!\Omega\!\!\text{ }$

Nên ${{\text{R}}_{\text{1}}}\text{=2}{{\text{R}}_{\text{2}}}\Rightarrow {{\text{R}}_{\text{1}}}\text{=2 }\!\!\times\!\!\text{ 4=8 }\!\!\Omega\!\!\text{ }$

Đáp án B