Answer A gives us an alternative reason for the conclusion in the argument, and thus weakens the logical connection.
That is: the conclusion of this argument is that warmer winters are probably responsible for the northern cardinal’s proliferation. But answer A gives an alternative explanation for why cardinals have proliferated since 1980 — far more common bird feeders, which are an important source of nutrition to wintering birds. This weakens the argument that it must have been the warmer winters.
