I don't know what passage you're talking about, but I think that A would be a better choice. To me, B seems a little too specific to be a main idea. However, if the passage never discusses the kindness of others then I think that B is probably correct.
It’s B (making a hasty generalization)
Answer:
the most common route taken by the stampeders to reach the fields was by boat from the west coast of the continental U.S. to Skagway in Alaska, over the Chilkoot or White Passes to the Yukon River at Whitehorse and then by boat 500 miles to Dawson City.
Explanation: