单选题 Weather exports use the ______ of the barometer as one of their guides in predicting the weather.

【正确答案】 B
【答案解析】(fluctuation波动，起伏)。句意为：气象专家把气压计的上下浮动，作为预报天气的依据之一。A．isobar等压线；C．gradation等级，阶段；D．mercury水银。