GMAT Critical Reasoning (CR) | Re: Two types of earthworm, one black and one red-brown, inhabit

Logic of GMAC is:
Before Event X: A was LESS than B.
After Event X: A equals B. OR A is larger than B.
Conclusion: X must cause A increase.
Assumption is: No factors make B decrease
I think that after the event X,A did not increase but B decreased due to predation so how can the conclusion be X must cause A increase ( although A might have increased through reproduction) At some point both A and B became equal
Can anyone please clarify!?

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