## PrepTest 79, Game 4, Question 22

### Transcript

Question number 22 is a global question. And it's asking, the spread of the virus among the computers is completely determined, if which one of the following is true? So the correct answer is gonna be one where if it's true, you're gonna know the complete virus chain. Now the best way to solve this question is to just go down the answer choices and try each answer choice.

If the answer choice presents you with a choice about how one of these rules plays out, then it's unlikely to be correct, because it doesn't seem like it's gonna be forcing you into a particular chain. The correct answer should be one where if you do what it say, then you're not left with a choice about how to apply the rest of the rules. You're gonna be forced into one virus chain.

So let's take a look at answer choice A, R transmits to Q. Now if R transmits to Q, that satisfies rule number 4. But do you notice that we still have no idea whether T goes to P or U goes to P? And because it seems like we still have a choice for who transmits to P, that means it's not going to completely determine the virus chain. The correct answers should force us into one of these possibilities, so A is not gonna be correct.

Answer choice B says T transmits to Q. And so that means we are in this version of rule number 4, T transmits to Q. But do you notice that we still don't know whether T transmits to P or U transmits to P? And it's also not even clear where this T-Q is gonna go. It's seems like it could go in the beginning over here or maybe at the end over here.

So because we have choices about where to put these computers, that proves that answer choice B is not going to determine everything. The next answer C says T transmits to S. And that means we're gonna have to put the T over here, because this is the spot that transmits to S. And if you look at the rules that mentioned T, that means we can't have T go to Q anymore, and we can't have T go to P anymore, because T already has the maximum two transmissions.

You can't have any computer transmitting to more than two other computers. So that means we now have to be in the world where R goes to Q, like this, and where U goes to P. So we still have to fit in a U goes to P. And do you notice, S needs to transmit to somebody. That's based on rule number 2, S must transmit to exactly one other computer, and U is the only one left that S could transmit to.

S would have to go to U, U would go to P, and we have just proven that answer choice C fully determines the virus chain. If T goes to S, then your chain has to look like this, and that's why it's the correct answer. Let's go ahead and prove why D and E are not correct. D says U transmits to P, and that means we're in this version of rule number 5.

But the problem is, we still don't know whether R goes to Q or whether T goes to Q. So because we have a choice of how rule number 4 plays out, that means answer choice D does not completely determine the virus chain. And finally, answer choice E says U goes to R, so that means U would have to go in this spot.

And because U is already transmitting to R and S here, it can't transmit to P. So we do know for this problem that T is gonna go to P, but I don't know how rule 4 plays out. You can have R go to Q right now, in which case the T would go to P over here, but it doesn't have to be like that. The R doesn't have to pass to the Q right now, the T could pass to the Q.

The T can pass to two other computers, and you could just put the T over here with P and Q like that. So that's why answer choice E does not determine everything.