The researchers extracted extremely fine particles containing nerve growth factor (NGF) from slow-soluble materials used for surgical wound sutures and mixed them with fetal brain cells containing cholinergic neurons. When the mixture was implanted in the brains of healthy adult mice, they found that NGF levels around the implant peaked after 7 days, and it did not return to normal levels until 3 weeks later. What is the correct thing to understand about the "mixture" in the line drawing sentence? ['Very fine particles of NGF and fetal brain cells.', 'Cholinergic neurons.', 'Cholinergic acetyltransferase.', 'Nerve growth factor and cholinergic neurons.']