Usage of 'fill in for'

[Andrey]

English Language Tests, Intermediate level

ESL/EFL Test #192 "Phrasal Verbs (D to F)", question 9

Karen had to ......... for Dave while he was away on vacation.

(a) fill up
(b) fill
(c) fill in

English Language Tests, Intermediate level

ESL/EFL Test #192 "Phrasal Verbs (D to F)", answer 9

Karen had to fill in for Dave while he was away on vacation.

Correct answer: (c) fill in

Your answer was: incorrect
Karen had to fill up for Dave while he was away on vacation.
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Could somebody explain it please?

[Mister Micawber]

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'Fill in for' = substitute for; take the place of'.
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Native English teacher at Mister Micawber's