GWD30套语法题 哪位n人帮忙解答一下

Q1:

The artist Renoir’s last word was “flowers,” spoken as they arranged a bouquet of roses just picked from his garden in a vase on his bedroom windowsill.

1. as they arranged a bouquet of roses just picked from his garden
2. as a bouquet of roses had been just picked from his garden and was being arranged
3. as a bouquet of roses just picked from his garden was being arranged
4. during the arrangement of a bouquet of roses just picked from his garden
5. while they arranged a bouquet of roses that had just been picked from his garden

   Answer: C

不明白 标准答案中怎么能出现being 呢？什么时候才能用being 呢？

同问？