Here is the passage again:
As he scrubbed the tile grout with a toothbrush and bleach. Nick vowed never
again to let the bathroom get so filthy.
You chose to use Punctuation Rule 2 to fix the problem:
Subordinate clause + , + main clause.
The correction looks like this:
As he scrubbed the tile grout with a toothbrush and
bleach, Nick vowed never again to let the
bathroom get so filthy.
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