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
Neutral substances are transported across the cell membrane by 
Select and explain. 
A. Porins, B. lonophore, C. Lipopolysaccharides, D. Diffusion
D: Diffusion
Answer: D: Diffusion 
Explanation: Neutral substances cross membranes based on their concentration gradient. That occurs by diffusion.