Read the following article and answer the question. Article: After several days' warm weather around the New Year's Day,a strong cold wave with a sandstorm from Mongolia attacked Tianjin Sunday afternoon. From 3 o'clock p.m.,the strong cold wave from Mongolia engulfed most areas of North China and the biggest wind-force was 7.Tianjin also suffered this year's first sandstorm with the cold wind whistling.The sky became dusky and the air was permeated with sand.By 11 o'clock at night,the wind became weaker and the sandstorm died down.From Monday morning,the northern areas had a big drop in temperature.It is learned that flu and the infection of the upper respiratory tract are easy to be infected due to the severe pollution by dust in the air.Relevant experts remind residents to pay attention to their health care. Question: We learn that _ from the passage. Answer:

Pick from: a). sandstorms result in the fall of temperature b). we can easily get infected by the dusty air c). it is usually warmer before sandstorms come d). sandstorms usually come around the New Year's Day
Answer:
b).