An engineer constructs a refrigerator having a cubical shape that is $2$ m on a side. If the bottom and the top are perfectly insulated. What is the minimum thickness of insulation $(k=0\mathrm{.}010\frac{W}{m}.K)$ that the engineer should apply to the side walls to ensure a heat load of less than $500$ W $,$ when the inner and outer surfaces are $-10$ C and $35$ C $.$

a$.54$ mm

b$.,140mm$

c$.14$ mm

d$.35$ mm