Polyvinylchloride automotive body panels (k = 0.092 W/m · K, c_p = 1.05 kJ/kg · K, ρ = 1714 kg/m³), 3-mm thick, emerge from an injection molder at 120°C. They need to be cooled to 40°C by exposing both sides of the panels to 20°C air before they can be handled. If the convective heat transfer coefficient is 30 W/m² · K and radiation is not considered, the time that the panels must be exposed to air before they can be handled is
(a) 1.6 min

(b) 2.4 min

(c) 2.8 min

(d) 3.5 min

(e) 4.2 min