be bad at 什么时候用at 什么时候用for

[A] bad at:
(1)They are bad at tennis (= they play tennis badly); bad at chess; bad at computer games...
[B] bad for:
(1)Smoking is bad for your health. 有害的
(2)Divorce is bad for children.
(3)The closure of the project is bad news for her staff.
(4)You are a bad boy for repeating what I told you.
(5)Too much sugar is bad for your teeth.
[C] bad about; badly off...
(1)She feels bad about how she treated you.
(2)He felt bad about leaving.
(3)We can't go on holiday – we are too badly off. 缺少的

　　be bad at意思为“不擅长”
　　be bad for意思为“对－－－没有好处”