A college recently conducted a survey on whether scholarships can promote learning efficiency. The survey includes three indicators of self-study attendance, the average time required to complete assignments, and the average daily reading volume. It is 25% more efficient than students who have not received scholarships. It is concluded that the effect of scholarships on helping students improve their learning efficiency is obvious. Which of the following would weaken the conclusions most? ? ['Those countries that receive scholarships are usually due to good study habits and high learning efficiency.', 'Scholarship students can more easily improve the learning environment to improve learning efficiency.', 'Students with low learning efficiency usually study long and lack normal rest.', 'Research on the relationship between the level of learning efficiency and the amount of scholarships should be conducted in a quantitative manner.']