eating habit与eating habits如何正确使用

我是一位六年级孩子的家长.近两天给孩子看作业,关于eating habit和eating habits的用法把我看糊涂了.以下是实例:

David asks the volleyball star,Cindy Smith,about her eating habits.引自:PEP七上:P25

How are your eating habits.引自:PEP七上配套练习[满分训练法]:P66.课后作业.能力.

good eating habits.出处:同2:P68重点短语.12

They have good eating habits.出处:同2:P68,核心句型,I.5

My father has a good eating habit,and he is healty.出处:同2:P70,IV.2

Does he have a good eating habit.出处:同2,P71,VI.2

Chinese change eating habits.网络文章

....And the eating habits of many Chinese have experienced significant changes during this period of time.网格文章

Sun liang has been in the Catering industry for more than 20 years,He's witnessed big changes in eating habits.网络文章.

Chinese eating habits will go hand in hand with China's reforms.网格文章

以上名子中,我没有看出有什么规律:泛指时用eating habits,确指是用eating habit?还是单数用eating habit,复数用eating habits?

希望有专业英语人士帮助解答.谢谢.

我的邮箱:transcend2008@126.com