题目选项分析

The industrial pollutants known as PCB's were first manufactured in 1929 and were used as coolants for electrical equipment in Europe and North America the 1970s, when studies showed the entry of the compounds into the food chain as harmful to some animals.

The industrial pollutants known as PCB's were first manufactured in 1929 and were used as coolants for electrical equipment in Europe and North America the 1970s, when studies showed that the compounds had entered the food chain and were harmful to some animals.