What is the difference between concerning and complacent?

[Thameem]

English Language Proficiency Tests, Advanced Level

ESL/EFL Test #91 "Advanced Vocabulary", question 4

You must react now and not just be ......... as if there is nothing wrong.

(a) complacent
(b) combining
(c) combative
(d) concerning

English Language Proficiency Tests, Advanced Level

ESL/EFL Test #91 "Advanced Vocabulary", answer 4

You must react now and not just be complacent as if there is nothing wrong.

Correct answer: (a) complacent

Your answer was: incorrect
You must react now and not just be concerning as if there is nothing wrong.
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what is the difference between concern and complacent? Please explain.

[Mister Micawber]

In this quiz question, 'concerning' is the wrong word form and holds the opposite meaning from the gist of the sentence. 'Unconcerned' is a possible answer.
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Native English teacher at Mister Micawber's