Scientists can trace the chemicals'effects,because some of the compounds remain in the environment for a long time. 因为一些化合物能在环境中长时间存留，使科学家能追踪这些物质的化学影响作用。
Cadmus, alarmed, prepared to encounter a new enemy, but one of them said to him, "Meddle not with our civil war. 说毕就挥剑刺死一个同他一起从土中长出来的兄弟。
