Many bacteria cannot easily be cultured from their natural habitat. Usually that is because those cells normally obtain a necessary metabolite from a host organism (or other member of the microbial community) and can no longer synthesize that requirement for themselves. Examples of such growth factors include all of the following except .A. amino acids (such as cysteine and proline). B. vitamins (such as riboflavin and thiamine). C. hormone-like peptides produced by other microbes. D. adenosine triphosphate. E. nicotinamide adenine dinucleotide (NAD) and derivatives.