@themoodyactivist I think in this stimulus, “after all” is the key clue because it introduces the evidence. The author’s main point is that not every mammal is suitable to keep as a pet. The tiger sentence is the support because it gives the reason why: tigers are aggressive and can seriously injure people.

We can also use the therefore test:

“Tigers are very aggressive and can cause serious injuries to people. Therefore, not every mammal is suitable to keep as a pet.”

That makes sense.

But the reverse does not work as well:

“Not every mammal is suitable to keep as a pet. Therefore, tigers are very aggressive and can cause serious injuries to people.”

That does not really follow, because the broad claim does not prove that specific fact about tigers. So the tiger sentence is the premise, and the broader claim about mammals is the conclusion.