Wonder vs. miracle

[Lyalya]

Test No. errors/inter-15 "Sleep well", question 8

She used it every day from then on and it was a wonder.

(a) used
(b) then
(c) wonder

Test No. errors/inter-15 "Sleep well", answer 8

She used it every day from then on and it was a miracle.

Correct entry: miracle
The error was: (c) wonder

You have not found the error.
She used it every day from then on and it was a wonder.
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Hello Alan and Torsten! Why word doesn't the word wonder fit in here?

[Torsten]

Because it's a fixed expression.

TOEIC listening, talks: A CEO is announcing his retirement

[Bhikkhu1991a]

Hello,