Researchers used multiple backward tasks to train one of the human brain functions, "working memory". All subjects were divided into four groups on average, no training group, 8 days, 12 days, 17 days training group All subjects (including the non-training group) received intelligence tests before and after training. The study found that compared with the non-training group, the intelligence level of the training group was significantly improved, and the longer the training time, the more the intelligence level improved. From this, it is concluded that "working memory" training can improve intelligence. Which of the following, if true, would most weaken the conclusion? ['The intelligence levels of the four groups of test subjects before training are different.', 'As the training time increases, the rate of intellectual improvement slows.', 'Individual "working memory" can be trained by other tasks.', 'Studies confirm that multiple retrogressive tasks can lead to improvements in other brain functions.']