The nurse is developing a plan to teach a mother how to reduce her baby's risk of developing otitis media. Which of the following directions should the nurse include in the teaching plan ？（）

A．Administer antibiotics whenever the baby has a cold.

B．Place the baby in an upright position when giving a bottle.

C．Avoid getting the ears wet while bathing or swimming.

D．Clean the external ear canal daily.

用电设备的工作电压一般允许在额定电压的（）范围内变动。

A.±2.5％

B.±10％

C.±5％

D.±5％。～10％