please explain

Stacks have been getting very expensive lately! I want to limit the stack size of a PDA to be at most 25 to save cost; call the machines that have this property RPDAs. This model works exactly as a normal PDA does, but if the stack has height 25 and on the next transition does not pop and does push, the computation immediately stops. What is the relation between the class of languages PDAs recognize and those that RPDAs recognize? They are exactly the same. They have no languages in common. Every language a PDA recognizes, a RPDA also recognizes, but not vice versa. Every language a RPDA recognizes, a PDA also recognizes, but not vice versa. There are languages in common, but some languages a PDA can recognize and a RPDA cannot, and vice versa