Is "step by step" an idiom?

[Steppy]

English Error, Elementary Level

ESL/EFL Test #23 "The bank raid", question 7

Sid explained pace by pace how they would get the money and told him when it would be and at what time.

(a) pace by pace
(b) get
(c) what

English Error, Elementary Level

ESL/EFL Test #23 "The bank raid", answer 7

Sid explained step by step how they would get the money and told him when it would be and at what time.

Correct entry: step by step
The error was: (a) pace by pace

You have found the error but your entry is incorrect.
Sid explained one by one how they would get the money and told him when it would be and at what time.
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why not?

Steppy

[Mister Micawber]

.
The idiom required is 'step by step'.
.
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Native English teacher at Mister Micawber's