很高兴为您
each year 在这里是副词短语,副词短语前不需要介词.
类似的表时间的副词短语还有this day/morning/afternoon/evening/month/year均可作时间副词短语.
We sell millions of airline tickets and room nights each year.
The United States is in a serious recession today.
I read this book this month.
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