The answer to this is DNA.
I think that the answer is C.
The answer would be D. Tropical Rainforest, this would be due to the abundance of rain which creates plenty of vegetation and then, more herbivores will live in the rainforest, and the more herbivores there are, the more carnivores will come to the rainforest. The canopy of the tall trees also provide shelter for countless of thousands of species of birds.
Provided that there's no diagram, most molecules have particular areas on the molecules, called the "active area" that act as a lock to a particular substrate's molecular structure, providing the key.
An enzyme's active area is only able to be "unlocked" by a certain substrate's "key".
