They're technically different things, but if you wanted to, you could diagram them as the same thing since "stopping deforestation" would by definition mean "forest not disappearing at current pace."
Mehran likely diagrammed them with different variables to better explain their relationship. That said, the key to answering this question is understanding that the biologist and politician's statements have different conditional relationships.
Does this make sense? The inference you made is correct, and whether or not you choose to diagram "stop deforestation" with the same variable as "forest not disappearing at current pace," you would still arrive at answer B because of the difference in the structure of the conditional statements that the biologist and the politician make.
Hope this helps. Let us know if you have any other questions!
