High school physics

First of all, it is clear that the spring supports the weight of the cylinder plus the gas in the cylinder. No matter the change of external air pressure or internal air pressure, the weight of the cylinder and the gas in the cylinder will not change, so the length of the spring will not change, so option A is wrong. When the external air pressure increases, the pressure of atmospheric pressure on the upper surface of the cylinder and the lower surface of the piston will increase because they can move freely. It is bound to compress the objects in the cylinder and achieve a new pressure balance, but the length of the spring will not change, so the cylinder will go down a little, so option B is correct and the external atmospheric pressure will increase. As you can see, you can use more hands. One hand presses the cylinder and the other hand presses the piston, and both hands will exert the same force. The gas inside will definitely be compressed and the volume will become smaller, but the length of the spring will not change, so the height of the cylinder will decrease.

With the increase of temperature, according to PV/T= constant, the external temperature will cause the internal gas temperature to increase, but the volume will increase to keep P unchanged (atmospheric pressure plus the weight of the cylinder unchanged), but the length of the spring will remain unchanged, so the cylinder will rise, so choose D instead of C.

Let's look at this problem again. This problem is actually an opposite change. At first, the external pressure increases, and then the internal pressure increases. If you can understand one, the other is the other. The confusing thing about this problem is that it is supported by a spring, not a stick, but after analysis, we know that it is the same as a stick, because the spring bears the weight of the cylinder plus the weight of the gas in the cylinder, and both.