Which of the following is not true about nonnative species?
a. they can have positive impacts on an ecosystem.
b. they can have negative impacts on an ecosystem.
c. they can be introduced within their natural range.
d. a nonnative species can be an invasive species.

c. They can be introduced within their natural range.

Invasive species can threaten an ecosystem and the biodiversity of the organism that exist in the present environment when the invasive species increase rapidly in number. 
Exotic species are a threat to biodiversity because they alter the ecosystem of that area. They share food and habitat resulting in unbalanced ecosystem. Also, habitat destruction destroys the habitat of all the life forms in that area.