Glass is largely transparent to incoming short- wavelength radiation from the sun (including visible light and some short-wavelength infrared). This radiation warms the interior of the greenhouse (ground, plants, air). The warmed objects then re- emit energy as longer-wavelength infrared radiation (heat). Glass is relatively opaque to this longer- wavelength infrared radiation, trapping the heat inside and causing the temperature to rise. Option A describes this process where shorter wavelengths enter and longer wavelengths cannot easily exit.