picnic前面什麼時候加冠詞

前面用冠詞the或者a，例如：

1、We stopped for a picnic en route.我們在途中停下來野餐。

2、We had a picnic beside the river.我們在河邊野餐。

3、He is ill， so he didn't take part in the picnic.他病了，所以沒參加這次野餐。

4、She was keen on going to the picnic.她很想參加這次野餐。