An iron rod of length `50cm` is joined at an end to an aluminium rod of length `100cm` . All measurements refer to `20^(@)C` . Find the length of the composite system at `100^(@)C` and its average coefficient of linear expansion. The coefficient of linear expansion of iron and aluminium are `12xx10^(-6)//^(@)C` and `24xx10^(-6)//^(@)C` respectively.
A. `36 xx 10^(-6 //@)C`
B. `12 xx 10^(-6 //@)C`
C. `20 xx 10^(-6 //@)C`
D. `48 xx 10^(-6 //@)C`