The latest report from an international paleontology research team stated that the South Turtles that lived in South Africa 280 million years ago were the ancestors of modern turtles, and they survived the mass extinction of the Permian to Triassic species. In order to avoid the harsh natural environment, they worked hard to dig downwards. At the same time, to ensure that the forelimbs' digging action was strong enough, the body needed stable support, which led to the continuous widening of the ribs. It can be seen that the turtle's shell is adapted to the performance of the environment is not just for protection, but for burrowing underground. Which of the following should be added as a prerequisite for the conclusion of the above conclusion? ['Modern tortoises have inherited some of the habits of Zhengnan turtles who are good at burrowing.', 'Only burrows can survive mass extinction.', "The turtle's shell evolved after the turtle's ribs gradually widened.", "Zhengnan turtle's forelegs are strong enough and do not require turtle shell protection."]