Explain "an all-time high"

[HuongItaly]

Business Idiom in English, Intermediate level

ESL/EFL Test #102 "Retirement Plans", question 8

Gas prices are at an all-time ..........

(a) interest
(b) high
(c) expensive
(d) rise

Business Idiom in English, Intermediate level

ESL/EFL Test #102 "Retirement Plans", answer 8

Gas prices are at an all-time high.

Correct answer: (b) high

Your answer was: incorrect
Gas prices are at an all-time interest.
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Dear teachers,

Could you please explain "an all-time high" ?
High in this case is a noun ?

Thank you !

[Mister Micawber]

Yes. An adjective turned into a pronoun: 'Gas prices are at an all-time high [price].'
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Native English teacher at Mister Micawber's