Meaning of 'as dry as a bone'

[Ilu]

Test No. express/elem-1 "Adding Up", question 8

You will not slip over because the floor is as dry as a bone.

(a) completely hard
(b) extremely flat
(c) totally dry
(d) completely cracked

Test No. express/elem-1 "Adding Up", answer 8

You will not slip over because the floor is totally dry.

Correct answer: (c) totally dry

Your answer was: incorrect
You will not slip over because the floor is completely hard.
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I can't unerstand what you mean by "dry". Out of
water? or simply uncomfortable?

[Torsten]

If something is as dry as a bone, there is no moisture there whatsover. So, in our instance there is absolutely no water on the floor which makes it very safe to walk.
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