10. Gateway Corporation reported sales of $180,000 for the current year. If accounts receivable increased $24000 during the year and unearned revenue decreased $4000 during the year, cash collected from sales was:

A. $152000

B. $156000

C. $160000

这个unearned revenue 怎么理解，为什么unearned revenue 减少了，cash collected from sales 就该减去它呢？不理解，请高手释疑解惑，谢谢！

自己顶一下，请高手帮忙解答一下，thanks！