When it gets cold and vegetation begins to become sparse, usually at the end of October, Canadian black bears go into hibernation. If an extended unseasonal cold snap arrives, however, the bears can go into hibernation as much as a full month earlier. Because bears are the major predator of salmon, river salmon populations are more robust when the bears are hibernating. However, this past year, when Canada experienced the coldest September on record, river salmon populations in October were much smaller than usual.

A When it is very cold, the number of salmon fishermen drops.
When it is very cold, the number of salmon fishermen drops.
B
Before going in to hibernation, bears tend to store fat for the long winter by eating as much vegetation as they possibly can.
C
When salmon populations grow, the herring populations shrink, since salmon prey on herring.
D
This past year, September was so unusually cold that some of the rivers froze solid.
E
Except in periods of extreme cold, salmon thrive more in slow-flowing rivers than in fast-flowing rivers.

The correct answer is C, but I think A is the correct answer.

My idea is When it is very cold, the number of salmon fishermen drops.( The first part of the passage doesn't say about when it is very cold, instead it says it in the end). So I think that very cold is the special case for decline in population of salmon. Answer C says some of the rivers froze. It doesn't say that rivers in Canada are froze.