576. eople 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？
以下是引用singdeath在2008-9-6 18:28:00的发言： 我认为D主要错在最后一个字they，跟未划线部分连起来读they forget them every time a new source。第一个they指people，第二个them指solar principles，指代不一。
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发表于 2004-2-26 18:19:00
eople 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.
