aeolusero二

LS, you answered your own question.

取非得到“BBB将成为唯一一家供应商”，这样的话BBB就能够接手其他退出的供应商的客户，因此销量和收入将不会大幅下降。

That supports the conclusion, instead of making it fall apart!

aeolusero二

Okay, I read your explanation wrong.

If you negate B) you have:
Soon the Brindon Bolt Barn might (or might not) no longer be the only significant supplier of fasteners to Brindon County’s workshops.

If this is true, BBB could have decreasing sales if it has no other clients. Or if BBB has other clients and expands its client base in other counties, it might avoid decreasing sales.

Therefore, the conclusion might still holds.  Thus, B) is not necessary.