576. People have discovered the principles of solar energy whenever fuel becomes scarce and expensive but will forget them every time a new source of cheap energy is developed. (A) have discovered the principles of solar energy whenever fuel becomes scarce and expensive but will forget (B) have discovered the principles of solar energy whenever fuel has become scarce and expensive but they forget (C) discovered the principles of solar energy every time fuel becomes scarce and expensive, forgetting (D) discover the principles of solar energy every time fuel became scarce and expensive, but they forget (E) discover the principles of solar energy whenever fuel becomes scarce and expensive but forget E 疑问：D为什么错？
