The thermal conductivity of a solid depends upon the solid’s temperature as k = a t+b where a and b are constants. the temperature in a planar layer of this solid as it conducts heat is given by: a. at+b= x +c2b. at+ b = c1x^2 +c2c. at^2+ bt= c1x+ c2d. at^2+ +bt= c1x^2+ c2e. none of these