'ideal' vs 'idea'

[Mudko]

English Error, Elementary Level

ESL/EFL Test #18 "Cat and mouse", question 10

At the police station the cat had a good ideal and smashed the glass on the floor. The cat was free at last and the mouse is now on the run.

(a) ideal
(b) on
(c) on

English Error, Elementary Level

ESL/EFL Test #18 "Cat and mouse", answer 10

At the police station the cat had a good idea and smashed the glass on the floor. The cat was free at last and the mouse is now on the run.

Correct entry: idea
The error was: (a) ideal
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explain how to error?

Mudko

[Mister Micawber]

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Choose which word (a, b or c) is NOT correct in the sentence. Here, 'ideal' is wrong and 'idea' is the correction to be made.
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Native English teacher at Mister Micawber's