So this distance right over here, let's just call this x of t, and let's call this distance right over here y of t. Now, how does the distance between the cars relate to x of t and y of t? Well, we could just use the distance formula, which is essentially just the Pythagorean theorem, to say, well, the distance between the cars would be the hypotenuse of this right triangle. Remember, they're traveling from perpendicular directions, so that's a right triangle there. So this distance right over here would be x of t squared plus y of t squared and the square root of that, and that's just the Pythagorean theorem right over here. This would be d of t, or we could say that d of t squared is equal to x of t squared plus y, let me, too many parentheses, plus y of t squared. So that's the relationship between d of t, x of t, and y of t, and it's useful for solving this problem because now we can take the derivative of both sides of this equation with respect to t, and we'd be using various derivative rules, including the chain rule, in order to do it, and then that would give us a relationship between the rate of change of d of t, which would be d prime of t, and the rate of change of x of t, y of t, and x of t and y of t themselves. And so if we look at these choices right over here, we indeed see that d sets up that exact same relationship that we just did ourselves, that it shows that the distance squared between the cars is equal to that x distance from the intersection squared plus the y distance from the intersection squared, and then we can take the derivative of both sides to actually figure out this related rates question.