This question is about an exchange reaction, specifically a double, which is also called a double replacement reaction. So we call it a double replacement reaction in this course, but some other courses would call it an exchange reaction. And we just want to make sure that you've heard that term. This is a multiple choice question. And this goes like this, which of the following compounds would result in an an a, double replacement reaction. It makes an an aqueous solution with lithium, whatever this thing is, and it turns out that's oxalate. This is going to be lithium plus. And since it takes two of them, even if we don't know what this is, it should have a two minus charge. And when we look at our solid body list, because what it means is we'll result in a double replacement reaction is we need there to be a solid wood result in a double replacement reaction means there is a precipitate a solid. So that's what the question means. Now let's go ahead and look through here. So first off, if you have oxalate, oxalate is on our list. Oxalate is on our solid body list right here. It says, except for rule one, oxalates are insoluble. So as we look down this rule number one says that anything with a group one or an ammonium will be soluble. Anything that does not have one of these, and I guess there are other group ones as well, but these are the most common ones. And because we've got a lithium here, anything with lithium is soluble. Let's go and do the first one here. So this one's going to be lithium oxalate. And you can see that our possible products are going to be lithium chloride, the positive from one with the negative of the other. And lead to oxalate. Bingo, that's going to be insoluble, which means it's going to be solid, which means that there is a double replacement reaction. Let's do the next one here. Ammonium. Oh, and we have the boxes and boxes mean there's more than one choice possible. So we have to look at all of them. Happy happy joy joy. Ammonium sulfate has ammonium in one of them. It has lithium in the other one. That means both combinations or any combinations of them will always be soluble, which means there is no precipitate and there is no reaction. So don't check that one. Cesium. I think I forgot to put cesium on my list of group ones here. There is cesium. It is in group one. If you have something with cesium and something with lithium, they will always be aqueous or soluble. There will never be a precipitate. So calcium. Let's see. Calcium. I was like to double check my solubility rules. Oxylates. Right. So there are no exceptions to this. Accept for rule number one. Right. Rule number six has except for rule one and calcium. But that's only for sulfides. Sorry if you can't read that. Let's get that over closer. So rule number five except for rule number one. Carbonates, hydroxides, all the way down to oxylates are insoluble. So that means calcium oxylate will be insoluble. And that's a little bit about how to do that one.