Meaning of a red rag to a bull

[Guest]

Test No. incompl/inter-2 "Like a red rag", question 1

Don't talk to him about politics because it's like a ......... rag to a bull.

(a) blue
(b) white
(c) red
(d) black

Test No. incompl/inter-2 "Like a red rag", answer 1

Don't talk to him about politics because it's like a red rag to a bull.

Correct answer: (c) red
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i need some information about
blue or red ... rag

[Alan]

Blue often has the meaning of sad or miserable. Red is used for danger and in this sense anger: like a red rag means something which will make him angry so don't talk about it.
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English as a Second Language
You can read my ESL story Relative Pronoun