Choose the correct answer:II. Have you ever thought, when looking through a window, that at one time there was not a pane of glass in the world? Then a man dug things out of the earth, mixed them, and heated them, and he found he had made something hard, smooth, and clear, so that he could see through it. Who was he? We do not know. We do know, though, that we owe him a great debt, and we owe a debt, also, to other discoverers and inventors whose names are no longer known.D. The names of other discoverers and inventors are no longer known because -