A study found that fresh vegetables were kept at room temperature and stored for two days in cold storage, and the nitrite content would rise, and to a higher degree than the national standard limit, causing mild to severe poisoning of the human body. A study found that fresh vegetables were kept at room temperature and stored for two days in cold storage, and the nitrite content would increase, and to a different extent than the national standard B value, which would cause mild to severe poisoning of the human body. Researchers immersed 5 kinds of long-lasting vegetables in fruit detergent for 5 to 10 minutes, rinsed them with water, and then tested the nitrate content. The results showed that the nitrate content in the vegetables decreased significantly and quickly decreased. Therefore, researchers believe that long-term vegetables can be safely used after being washed with fruit and vegetable detergents. Which of the following, if true, would weaken the researchers' experimental conclusions most? ['Different kinds of nitrite in 5 kinds of vegetables.', 'Chemicals in fruit and vegetable detergents can damage the taste of vegetable.', 'c. The nitrite content in the experiment only decreased for a short time, and soon returned to the state of exceeding the standard.', 'Soaking in clean water for more than 20 minutes can also achieve the same effect in the experiment.']