Sentence Correction

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

Ans is E

ABC is incorrect because of tense (past and present, not cosistent)
Why is D wrong -use of they at end, ambigious at the end.

Thanks
khurram