She was doing her homework one Sunday morning when she smelt something burning.

She was doing her homework one Sunday morning when she smelt something burning.
She was doing her homework one Sunday morning when she smelt something burning.

She was doing her homework one Sunday morning when she smelt something burning.
在一个星期天的早晨,正当她在做作业的时候闻到了有东西烧焦的味道.

星期天的早上，她正在做作业的时候，她闻到了有什么东西烧焦的味道

星期天早上当她在做作业的时候，她闻到了东西烧焦的味道。

一个星期天早上，她正在做作业，这时她闻到了有什么东西烧焦的味道。