What does this idiom mean: "as dry as dust"?

[Rich7]

Test No. incompl/advan-67 "Dry as you like", question 3

She studies some very obscure things to do with ancient Greek documents - to most people a really dry as ......... subject.

(a) powder
(b) earth
(c) dust
(d) soil

Test No. incompl/advan-67 "Dry as you like", answer 3

She studies some very obscure things to do with ancient Greek documents - to most people a really dry as dust subject.

Correct answer: (c) dust

Your answer was: correct
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could you explain this a litle it?

[Torsten]

Something as a dry as dust is very boring or dull.
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Test Of English for International Communication
TOEIC Preparation & TOEIC Vocabulary

[Rich7]

Thanks,