Respuesta :
Hi !
A- She leaned her bike against a tree, but the bike wouldn't stand up.
B- She leaned her bike against a tree, but it wouldn't stand up.
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A and B are both correct grammatically.
A repeats twice the word "bike" and you don't have to over-use the word since you can replace it by the personal pronoun "it" (B).
Unless, you want to insist on the noun "bike", for some reasons....
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