6. Given are four sentences on a topic. Some sentences are grammatically incorrect or inappropriate. Select the option that indicates the grammatically correct and appropriate sentence(s).
A. A quantum cascade laser works by pumping up electrons.
B. But instead of dropping back in a single step, its fall is controlled so that they give up their energy a bit at a time.
C. The electron cascades down as it moves through the laser, which is built from thin layers of different semiconductors.
D. The amount of energy given up at each step, and hence the wavelength of the radiation given off, is governed by the thickness of the layer, not the properties of the material.