May be challenged or have been challenged?

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Hi again all grammar geeks ! Please explain to me why "may be challenged" is wrong in this test:

Great as Newton was, many of his ideas .......... today and are being modified by the work of scientists of our time.

(a) are to challenge
(b) may be challenged
(c) have been challenged
(d) are challenging

Why is only (c) correct?

Thousand thanks!

[Mister Micawber]

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'May be' states present possibility, but that is not a reasonable statement by the writer, since the following clause erases the mere possibility and presents the fact of challenge ('modification').
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Native English teacher at Mister Micawber's

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Thank you, Mr! I got it !