Quiet as a mouse

[Richard]

Test No. incompl/inter-42 "Keeping it Quiet", question 1

As it was very late when he came home he was as quiet as a ......... in order not to wake anybody up.

(a) cat
(b) mouse
(c) dog
(d) rat

Test No. incompl/inter-42 "Keeping it Quiet", answer 1

As it was very late when he came home he was as quiet as a mouse in order not to wake anybody up.

Correct answer: (b) mouse

Your answer was: incorrect
As it was very late when he came home he was as quiet as a rat in order not to wake anybody up.
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Hi!
Why the answer RAT is incorrect?

[Alan]

Hi,

This is the comparison that is used and cannot be explained gramatically.

Alan
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English as a Second Language
You can read my ESL story Colour Idioms