【单选题】
由厚度都相同的平壁组成的三层平壁而言,若λ<sub>1</sub>>λ<sub>2</sub>>λ<sub>3</sub>,则热阻R<sub>1</sub>,R<sub>2</sub>,R<sub>3</sub>之间的关系为()。 
A、R1>R2>R3
B、R1<R2<R3
C、R1>R3>R2
D、R3>R1>R2
A、R1>R2>R3
B、R1<R2<R3
C、R1>R3>R2
D、R3>R1>R2