Meaning of "clammed up"

[Meni Saar]

English Language Tests, Intermediate level

ESL/EFL Test #191 "Phrasal Verbs (C)", question 5

He ......... when the police officers started to question him about the robbery.

(a) clammed
(b) clammed up
(c) clammed about

English Language Tests, Intermediate level

ESL/EFL Test #191 "Phrasal Verbs (C)", answer 5

He clammed up when the police officers started to question him about the robbery.

Correct answer: (b) clammed up

Your answer was: incorrect
He clammed when the police officers started to question him about the robbery.
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Correct me if I'm wrong, is clammed up means frightened or nervous?

Meni Saar

[Canadian45]

No, ''clammed up'' means he stopped talking. He wasn't willing to say anything.