IMO - D, because excessively restrictive controls would be created when they create controls on things that don't actually get to the North sea.

Think of almost all fill in the blanks question in GMAT as assumption question.

What is the assumption required to have controls on quality of effluents going to North sea?

Option D explains the required assumption correctly.

IMO B -
I think the key word here excessively restrictive controls
to avoid excessive controls --> make sure the substance can actually cause damage.
If the substance does not cause damage -> it does not need to be controlled

I think it's B. The problem said that they favor uniform controls regardless of whether damage can be traced to a particular *source* (i.e. whether or not it can be shown that Factory A is the source of a chemical that caused specific damage). But if the goal is to avoid environmental threats, then they should make sure that what they're controlling actually does damage.

They don't need to prove that *all* of the effluent reaches the North Sea. What if a factory is pumping out a thousand gallons of toxins a day, and only 999 gallons reach the North Sea? They'd still need to impose controls. So it can't be D.

I also think that it's B