In the 1960s, the University of California, Berkeley conducted a series of experiments on the cognitive capabilities of mice. In the experiment, one group of mice was placed in a cage full of toys, runners and tunnels, and the other group The mice grew up in empty cages. The experimental results showed that the brains of the former group were larger and their cognitive ability was significantly higher than the latter group. The researchers therefore reasoned: sufficient external stimulus Can alter the structure and capabilities of the brain. Which of the following is true to best strengthen a researcher's inference? ['In the experiment, the food of the former group of mice is more nutritious.', 'After the experiment, the latter group of mice is more athletic.', 'Later, the average lifespan of the two groups of mice was similar.', 'Before the experiment, the individual differences between the two groups of mice were not significant.']