大全908 介短修主还是谓

908. Upset by the recent downturn in production numbers during the first half of the year, the board of directors, at its quarterly meeting, raised the possibility of adding worker incentives

这是E答安的样子。

但我所划线的部分，我不明白； 介词短语, at its quarterly meeting,修饰主语还是谓语；为什么要用逗号呢？