Hey Guys,

I can take this on in a bit more detail.

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 until the 1970s, when studies showed that the compounds had entered the food chain and were harmful to some animals.

A that the compounds had entered the food chain and were harmful to some animals
B the compounds to have entered the food chain and be harmful to some animals
C the entry of the compounds into the food chain as harmful to some animals
D the entry of the compounds into the food chain and its harmfulness to animals
E the compounds entering into the food chain and harming some animals

The big issue is that we NEED the word "that" after show, because it's introducing a clause (phrase with a verb in it). So that gets us to (A) right away. More stuff.:

B) parallel broken between "to have entered" and "be harmful"
C) you can't "show X as Y". There needs to be a verb in there, and once there is, you need THAT.
D) same as (C), plus there's no intelligible antecedent for "its"
E) actually doesn't require THAT, because there's no verb, and the two -ing words are parallel. But the meaning is busted. The study didn't show the compounds entering into the food chain and harming animals (you'd need, like, a video of that actually happening). The study showed THAT the stuff happened.

Hope that helps!

-t