Reinvent the wheel

Test No. [color=blue]incompl/advan-77 “Internet Business (2)”, question 3

She told him to stop wasting time by coming up with tried and tested ideas and in this way simply re-inventing the .

(a) circle
(b) round
© curve
(d) wheel

Test No. [color=blue]incompl/advan-77 “Internet Business (2)”, answer 3

She told him to stop wasting time by coming up with tried and tested ideas and in this way simply re-inventing the wheel.

Correct answer: (d) wheel

Your answer was: [color=red]incorrect
She told him to stop wasting time by coming up with tried and tested ideas and in this way simply re-inventing the circle.
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I know there is a metaphorical idea which I can not comprehend.

This expression means going back to what has already been created/invented - in other words wasting time.