人体内缺少下列某种元素会造成食欲不振,生长迟缓,发育不良,该元素是( )
A:碘
B:铁
C:钙
D:锌
Old people should to politely.
DB是⊙O的切线,D为切点,过圆上一点C作DB的垂线,垂足为B,BC=3,sin∠A=34,则⊙O的半径为____ .
张霞在日记里写道“放学时,铃声是悦耳的;考试时,铃声是紧张的;思考时,铃声是刺耳的,”这说明( )