picnic前面什么时候加冠词

前面用冠词the或者a，例如：

1、We stopped for a picnic en route.我们在途中停下来野餐。

2、We had a picnic beside the river.我们在河边野餐。

3、He is ill， so he didn't take part in the picnic.他病了，所以没参加这次野餐。

4、She was keen on going to the picnic.她很想参加这次野餐。