In order to study the relationship between "light fasting" and human health, an institution recruited a group of volunteers for experiments. The volunteers were divided into two groups and the test period was 6 months. Fasting was continued for 5 days in the middle of the month. On the first day, the caloric intake was reduced to half of the recommended amount. After that, the daily intake was only one-third of the recommended amount for the 4th day. Both had a normal diet. The test results showed that the physical conditions of the first group of volunteers were significantly improved. From this, the researchers concluded that "light fasting" caused more ketone bodies, oxidative stress and inflammation marker levels in the volunteers. Both have declined. Which of the following, if true, would weaken the researchers' conclusions most? ['The physical function and stress response of the recruited volunteers are different.', 'The body inflammation of a volunteer who has undergone continuous fasting has been greatly alleviated.', 'Physical examinations of volunteers before the start of the trial showed that the relevant health indicators were basically similar.', "A certain group of volunteers' health level declined after the trial."]