diet前面可以用a 关于diet 的词组有?

She is on a diet.为什么diet前面可以加a呢? on diet ,on a diet都可以吗 英语词组解释下being on a diet have a balanced diet的中文意思 eat a balanced diet和have a balanced diet 可以互换吗? a healthy diet A Healthy Diet作文 a good diet go on a diet eat a balanced diet a balancing diet 和 a balanced diet 都是正确的吗? 英语不定冠词go on a diet diet 前面为何加不定冠词 不是名词可数才加不定冠词,