A 26-year-old primigravida is in labor. Her cervix is 5 cm dilated and 75% effaced; the fetus is at 0 station. The client requests medication to relieve the discomfort of contractions, and the physician prescribes an epidural regional block. What position should the nurse help the client to assume when the epidural is administered ？（）
A．Lithotomy.
B．Supine.
C．Prone.
D．Lateral.