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Well, your question about the use of the hyphen is that it draws a parrallel between the first half of the sentence and the second half:
... they will be able to make computers that can understand...
... computers that will be used to diagnose...
Like an expert must be used to compare similar pronoun
(D) and (E) do not draw this parrallellism and futhermore, in (D), it is not clear what "the use of which" refers to - does it refer to experts or computers which are used. (E) sounds as if the experts are to be used.