There are also differences between submarines and fish maws. Please tell me what submarines can change and what remains unchanged.

This problem is not a problem. Indeed, fish change their buoyancy by changing their volume. It is true that submarines are lifted by releasing water or releasing water, but this fact depends on how you understand it. You can understand that the submarine is constant and its buoyancy is certain. When it enters the water, it changes its own weight and controls the ups and downs. But think about it from another angle. When it is filled with water, the empty warehouse is also filled with water. Isn't it equivalent to less water? Isn't it equivalent to the constant gravity and the change of buoyancy? This is actually like a hemisphere, with its mouth floating in the water, its edge higher than the water, and it is filled with water. In fact, this question is strictly problematic: what does a submarine mean? Is it the whole of iron, musical instruments and people, or does it refer to everything in Tito's shell? This question is like this, different people have different understandings, but the usual education forces you to think that it is to change gravity. Personally, according to the second explanation, the understanding of the concept, the displacement has changed and the gravity has remained unchanged, which is indeed the same as fish. But in fact, there is no water in the fish's stomach! The principle of fish may be more like a balloon in water, and its volume is getting bigger and bigger. The principle of submarine is more like a hemisphere, in which water is added. I think the difference lies in this: the hemisphere can't replace its own equal volume of water, but because of its shape, it can replace the super volume of water, and then you add water to it, and the super volume is full of water. Finally, it tends to replace only the original volume of water (my hemisphere here refers to this ball, excluding water! The explanation in the book is that the hemisphere and water are called the same hemisphere, which shows how your problem came about. ) Fish can always discharge water equal to its volume, but the volume keeps increasing. So strictly speaking, it is different in essence, but it seems to be the same in principle. The reason for this difference is that we also count the excess volume as the displacement, and the equal volume displacement of fish is also called the displacement. From here on, the difference becomes the same. Do you understand now? However, the exam is still scripted! You can know for yourself, but you have nothing against each other! Junior high school exams are often written like this, which led me to think about this problem carefully and finally figured it out.