Agreed those are so tricky, especially since we were given similar examples a few weeks ago and told they were not arguments (as maald4c said). I wish I remembered what they were so that we could compare and contrast.

For me I found it helpful to think of what the author is trying to convince us of. The premise, "environmentalists have discovered that since the industrial revolution, there has been an increase in air pollution" seems to be there to solely provide context.

Meanwhile, the conclusion ("it seems that almost all of this increase resulted from fossil fuel combustion and industrial processes") is the author trying to convince us why the prior statement is true. So in this case, the premise/conclusion are more in context/hypothesis roles.

This is a really good question and I'm glad you brought this up! From this exercise, it seems to come down to how the premises are used to support arguments. Take 2.4 for example:

Fines for driving at high speed are now so high that it costs a driver more to pay the fine than it would have cost to travel safely [premise 1]. Therefore, since drivers value their licenses and safety [premise 2], those that might speed will now drive at safe speeds.

We see that the second premise makes an assumption from the first premise to arrive at the conclusion. In contrast, the premise in 3.1 cannot be broken down to the point where one is a statement and one is an assumption leading to the conclusion. Therefore, its taken as one premise. I think sentence structure might also play a role here but I'm not sure how relevant that is.

Would love to join this!