the wrong answer in exercise

The third answer is not true when it is not the same column as well as the total number of people in each group.

The following example is another proof.

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The answer (C) is actually correct. Unfortunately, Khan Academy, as far as I'm aware, doesn't have videos or articles that directly talk about these problems. The answer explanations are very unhelpful as well. The thing to note here is that it is a column relative frequency table. This means that, for example, 54% of people with Internet access prefer text messaging, while 31% of people without Internet access prefer text messaging (hence, column relative frequencies, referencing the fact the the columns represent having Internet acces, but not preferred communication methods). However, you cannot say that 31% of people who text message have no Internet access since the 0.31 is referencing the column "No Internet access", not "text messaging". So, you can make an association with having Internet access and preferred communication method, but not the other way around.