Idiom: loud and clear

[May Thant Zin]

Test No. errors/elem-14 "Listen and Improve Your English", question 5

At this precise moment you are my listener and I hope you can hear me loudly and clear.

(a) are
(b) loudly
(c) clear

Test No. errors/elem-14 "Listen and Improve Your English", answer 5

At this precise moment you are my listener and I hope you can hear me loud and clear.

Correct entry: loud
The error was: (b) loudly

You have not found the error.
At this precise moment you are my listener and I hope you can hear me loudly and clearly.
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I'd like to know why the answer is "loud and clear". These words are adj. I think the answer should be "loudly and clearly" in order to be adverb. I wonder if you explain more. Thanks.

[Mister Micawber]

Loud and clear are both adverbial forms as well, as especially used in this idiomatic collocation loud and clear, which means unequivocally:

loud and clear

Easily audible and understandable. For example, They told us, loud and clear, what to do in an emergency, or You needn't repeat itI hear you loud and clear. This expression gained currency in the military during World War II to acknowledge radio messages ( I read you loud and clear) although it originated in the late 1800s.

(Courtesy of Dictionary.com)
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Canadian-American native speaker
who teaches English for a living at Mister Micawber's
ESL cafe: Interview with Mister Micawber