Please help me explain the answer for this question.

[Hanglib]

This question is on grammar section of SAT.

Those inventors who sold stocks just before the stock market crashes in 1929 were either wise or exceptional lucky. No error

The mistake in "exceptional" word but I do not understand why. Please help me make clear.

[Beeesneees]

An adverb is needed. It should be 'exceptionally'.
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"Inside every old person is a young person wondering what happened."
Terry Pratchett