During afternoon rounds, the nurse finds a male client using a pencil to scratch inside his knee-to-toe cast. The client is complaining of severe itching in the ankle area. Which action should the nurse take ？（）
A．Allow him to continue to scratch inside the cast with a pencil.
B．Give him a sterile metal object to use for scratching instead of the pencil.
C．Encourage him to avoid scratching, and obtain an order for diphenhydramine (Benadryl) if severe itching persists.
D．Obtain an order for a sedative, such as diazepam (Valium), to prevent him from scratching.