= Solution
{c}
In a conventional <superconductor>, an attractive pairing interaction forms <Cooper pairs> and a phase-coherent condensate. The elementary fermionic excitations have a <superconducting energy gap> $\Delta$; breaking a pair into two excitations costs at least $2\Delta$. The density of thermally excited <quasiparticles> is exponentially small for $k_BT\ll\Delta$. Nevertheless the vanishing DC <electrical resistivity> below the critical temperature is a property of the coherent supercurrent, not a claim that all thermal quasiparticles disappear. Below the relevant critical-current and critical-field limits, a homogeneous bulk sample carries dissipationless current; above the superconducting transition the normal resistivity returns.
In a <semiconductor>, the <band gap> separates filled valence states from empty conduction states because of the periodic crystal potential and orbital hybridization. Thermally exciting carriers across the gap produces both conduction <electrons> and valence <holes>. For an intrinsic sample,
$$
n=p\propto T^{3/2}e^{-E_g/(2k_BT)},\qquad\sigma=e(n\mu_e+p\mu_h).
$$
Thus its <electrical resistivity> generally rises rapidly on cooling, with an activated factor $e^{E_g/(2k_BT)}$ up to mobility and density-of-states prefactors. This differs from the zero-resistivity condensate response. Dopant freeze-out and extrinsic-carrier regimes require their own activation scales; the intrinsic formula is not universal for every doped sample.
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