Because with a four-sided die, we're assuming is a four. So it's essentially, this is analogous to saying when you roll a six-sided die, what's the probability that you get a four as well? Because that's the only way you're going to get doubles given that the four-sided die is four. And we see that right over here. The six-sided die has to be a four as well in order for this to be doubles because we're assuming where it's given that B, we're given of event B. We're restricting our sample space with event B. What is the probability of B given A?