spontaneous

他们决定来一场说走就走的旅行。

They made a ___spontaneous decision to go on a road trip.

这种植物的开花过程是完全自发的。

The flowering process of this plant is completely ___spontaneous.

孩子们在公园里自发地开始了一场即兴的足球比赛。

The children started a ___spontaneous soccer game in the park.