The old lady has developed a ______ cough which cannot be cured completely in as hort time.
 
【正确答案】 C
【答案解析】 chronic“慢性的”,常用来形容持续时间长的病症。perpetual永久的,主要指时间。permanent永久的,持久的,强调事物固定不变。sustained持续不变的,强调不间断性。