tracer agent

例句：

1. 1.This tracer agent is used to locate the lost items.
   这种追踪剂用于定位丢失的物品。
2. 2. The scientist used a tracer agent to study the movement of water molecules.
   科学家使用一种追踪剂来研究水分子的运动。
3. 3. The tracer agent was injected into the river to track the pollution source.
   将追踪剂注入河流以追踪污染源。