Bimetals

Bimetal refers to an object that is composed of two separate metals joined together. Instead of being a mixture of two or more metals, like alloys, bimetallic objects consist of layers of different metals. The above left diagram [A] shows a bimetal consisting of two metals $X$ and $Y$ being heated up. The above right diagram [B] shows a bimetal consisting of two metals $Z$ and $W$ being cooled down. Now, the linear expansion coefficients of iron, copper and aluminium are $12 \times 10^{-6} \text{ /K}$ , $16 \times 10^{-6} \text{ /K}$ and $23 \times 10^{-6} \text{ /K}$ , respectively. Which of the following combinations will bend the bimetals upwards (resulting in the right ends lifted up as in the diagram)?

a) X=iron, Y=copper in the bimetal [A].

b) X=aluminium, Y=iron in the bimetal [A].

c) Z=aluminium, W=copper in the bimetal [B].