The alternate discs of iron and carbon, having same area of cross-section are cemented together to make a cylinder whose temperature coefficient of resistivity is zero. The change in temperature in two alternate discs is same. The ratio of their thickness and ratio of heat produced in them is found out. The resistivity of iron and carbon at `20^@C" are "1 xx 10^(-7) and 3 xx 10^(-5)Omegam` and their temperature coefficient of resistance are `5 xx 10^(-3)""^(@)C and -7.5 xx 10^(-4)""^(@)C,` respectively, Thermal expansion is neglected. Here, `triangleR_1 +triangleR_2= 0` (where `triangleR_1, and triangleR_2`, are the increase in resistances of iron and carbon, respectively, with the rise in temperature) because combined temperature coefficient of resistivity is given as zero.
Ratio of their thickness is

Options

A. 54
B. 45
C. 35
D. 21

Answer

B