Anwer should be D because:

We need to take into account the fact that even though the sulphur dioxide gases were eliminated while growing the plants, the urban plants grew slowly. Therefore the questions is what could be the other reason for their growing slowly.

The only point in the options that points towards the urban environment is D.

Hope my reply is convincing.

I will go with B ...

To evaluate how much difference in growth is due to sulfur dioxite we should compare the growth in presence of dioxite with the growth in absence of dioxite, so B does exactly that.

What is the OA?

I will go with D too...

See here sulphur dioxide was eliminated then the plants should have grown equally if there was no other impact.

But there was another reason why plants could be growing slowly other than sulhur dioxide and that is explained by D

So effect of Sulphur dioxide can be understood only if this is also eliminated.

Hope that makes sense.

I got it

I think D is the right answer here