Idiom: in cold blood

[Guest]

Test No. incompl/inter-18 "Hot and Cold", question 7

The accused was sentenced to life imprisonment after it had been established that he had committed the murder in cold ..........

(a) attack
(b) feeling
(c) blood
(d) passion

Test No. incompl/inter-18 "Hot and Cold", answer 7

The accused was sentenced to life imprisonment after it had been established that he had committed the murder in cold blood.

Correct answer: (c) blood

Your answer was: correct
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why is 'blood' not the others word?
could you explain thanks a lot!
are there some idioms?

[Mister Micawber]

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Like much of English, there is no explanation other than that 'in cold blood' is an idiom, meaning with premeditation, deliberately.
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Canadian-American native speaker
who teaches English for a living at Mister Micawber's
ESL cafe: Interview with Mister Micawber