Why is it considered partial support and not just support?

Something to note. AC A is an excerpt in the passage before Jazz was ever mentioned, so it's not possible for the author to have an attitude towards Jazz if it was never brought up in the text yet.

@Colin Erickson You're a stud

Is the assumption "Tigers are mammals" more reasonable based off our outside knowledge of this to be true, or is it because it's simply less vulnerable to criticism than the other assumption?