（-180）+（+20）
=-（180-20）
=-160．
依据中暑症状的轻、重分为（）三种类型。
A.先兆中暑
B.轻度中暑
C.中度中暑
D.重度中暑
Uncle John is getting old and he has trouble _____ what others say to him. [ ]
A. hearing
B. hear
C. to hear
