The correct answer would be A.secondary succession. I hope this helps.
The studies are designed this way b/c it removes the power of suggestion and the double-blind study keeps both researchers and participants in the dark. So this compared to who is receiving the treatment which is relevant b/c it stops the researchers from accidentally overbalancing the study participants, or unintentionally favoring their assessment of the conclusions.
I have a feeling its b but probs wrong so would double check
(12g)/(75.398cm^3)
=0.159 g/cm^3
answer would be = 0.16 g/cm^3
