trace of rainfall

例句：

1. 1. The trail leading to the waterfall is well-marked with footprints of previous visitors.
   通往瀑布的小径上，有前人留下的脚印。
2. 2.The raindrops on the windowpane are a testament to the recent rainfall.
   窗玻璃上的雨滴证明了最近下了雨。
3. 3.The scientist used a trace of water from the river to help identify the source of the pollution.
   科学家用河水中的一滴水来帮助确定污染源。