Total radial resistance per unit length is $\ln(r/r_i)/(2\pi k)+1/(2\pi rh_o)$. Differentiate with respect to outer radius and set to zero: $1/(2\pi kr)-1/(2\pi h_or^2)=0$,giving **$r_{cr}=k/h_o$,B**. The source calls this thickness, but the expression gives critical outer radius; added thickness would be $r_{cr}-r_i$.
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