Given, l= length of both the metallic stripes, t= thickness of both the metallic stripes,let l_1 and l_2 be the final length of the metallic stripes afterexpansion due to \\Delta T change. \\Delta l=l_2-l_1=(l\\alpha_2\\Delta T-l\\alpha_1\\Delta T) We know, l=\ heta r \\Delta l=\ heta \\Delta r Here, \ heta is constant as same arc angle is made by both the stripes.and, \\Delta r=t then \ heta=\\dfrac{l(\\alpha_2-\\alpha_1)\\Delta T}{t}=\\dfrac{l}{r} r=\\dfrac{t}{(\\alpha_2-\\alpha_1)\\Delta T} Hence option \ extbf B is the correct answer.