This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Bacteria, Subject name: Microbiology
[{'content': "Mycoplasmas differ from chlamydiae in that they are \nSelect and explain. \nA. Susceptible to penicillin, B. Able to grow on artificial cell-free media, C. Able to cause urinary tract infection, D. Able to stain well with Gram's stain", 'role': 'user'}, {'content': 'Answer: B \nExplanation: Mycoplasmas lack a rigid cell wall and are bound by a triple-layer unit membrane. For this reason, they are completely resistant to the action of penicillins. Unlike the chlamydiae, they can replicate in cell-free media.', 'role': 'assistant'}]