In order to verify the effect of a new longevity drug, the researchers conducted experiments with two groups of white rabbits. They injected the new drug with two groups of indifferent white rabbits, and then kept one group in a cage and the other group. Breeding in the natural environment. The results showed that the average life expectancy of the white rabbits raised in the natural environment was 1/10 longer than that of the white rabbits raised in the cage. The researchers believe that the relaxed environment is conducive to the function of the new drug. Which of the following is most likely the hypothesis that the researchers reached? ['A little white rabbit in a cage is not happy.', 'Little white rabbit lives easier after injection of new medicine.', 'Little white rabbits raised in the natural environment live easier.', 'The function of the new drug is closely related to the living environment of the subject.']