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
Mycoplasmas differ from chlamydiae in that they are 
Select and explain. 
A. 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
B: Able to grow on artificial cell-free media
Answer: B: Able to grow on artificial cell-free media 
Explanation: 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.