A 15 m² exterior wall must be insulated so that the conductive energy loss does not exceed 500 W when the indoor–outdoor temperature difference is 30 K. Two candidate insulation materials are available:
• Material A: thermal conductivity 0.035 W m⁻¹ K⁻¹, cost $45 per m³
• Material B: thermal conductivity 0.020 W m⁻¹ K⁻¹, cost $70 per m³
Assuming the entire wall is uniformly filled with one material, which material meets the heat-loss requirement at the lower total cost? State the material letter only.