#HELP

I'm still lost on how "strengthens the GROUNDS ... FOR the conclusion" = "strengthen the conclusion".

Conclusion: this rubbish by itself actually tells us relatively little about those possessions

Grounds: among other reasons, the pits have been subject to erosion over long periods of time, with destructive effects on the rubbish within

What strengthens the GROUNDS that the pits were subject to erosion over long periods of time?

C: The soil surrounding the rubbish pits was sometimes removed for the manufacture of bricks.

Removing soil is literally erosion. Although D also includes an erosion event (digging burial sites), it specifically says "earlier", while C does not have a timeline, making it more plausible that it occurred "over long periods of time".

I'm JUST now understanding that context outside of the premise does not actually add to the argument and can be ignored in these types of questions..! Thank you!