Until quite recently, American presidents lived in a world in which the public and private realms of their lives were largely seperate, and the press cooperated in maintaining the distinction, and Americans judged national leaders without receiving, or expecting, intimate information about them.
A..
B. where the press cooperated in maintaining the distinction, and where
C. for the press cooperated to maintain the distinction and
D. the press cooperated to maintain the distinction, for
E. in which the press cooperated in maintaing the distinction, and in which

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. ...
B. have discovered the principels 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

==> is there a difference between "discovered" and "have discovered" (couldnt you use the present participle because people could have discovered from the past into the present?
Also,...if you add "they", after "but" so it becomes "but they forget" (like D), is it wrong?