Hi CristinaCP!
You ask,
So my question is, how should I approach a broad inference question like this next time? What if I don’t remember or can’t find one of the details which would lead to an inference, and mistakenly conclude that an AC has no support? Do I just need to read more carefully?
Without knowing more about how you are approaching reading comprehension, I'd say it's not necessarily a matter of reading more carefully. That perhaps might partly be the case, but if you read
too carefully, that can eat up a lot of time and thus could make a difference for the worse in terms of overall score (e.g., reading too carefully might mean a student only gets to three passages rather than all four).
Reading with care is important, but it's almost equally as important to remember to notate and mark up the passage. Having arrows, plus and minus signs, numbers for lists, etc. can be very useful for knowing at a quick glance what is happening in each paragraph. Then when you move on to questions, hopefully those notations in the margins will direct you where to look.
For instance, you might have circled the mention of a "typical study" in the third paragraph, and drawn an arrow to the fourth paragraph with a minus sign next to it to indicate difference--that is, there is some key difference between the typical study and the study in the final paragraph. One difference that stands out is that successful dowsers aren't represented well in the typical study but they do appear to be represented in the study in the final paragraph.