This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Physiology
[{'content': 'Neutral substances are transported across the cell membrane by \nSelect and explain. \nA. Porins, B. lonophore, C. Lipopolysaccharides, D. Diffusion', 'role': 'user'}, {'content': 'Answer: D \nExplanation: Neutral substances cross membranes based on their concentration gradient. That occurs by diffusion.', 'role': 'assistant'}]