instruction:
Given scientific fact as input, generate the question from this fact such that it can be answered from the input.
question:
Saturated hydrocarbons hydrocarbons contain only single bonds between carbon atoms.
answer:
What hydrocarbons contain only single bonds between carbon atoms?


question:
The nuclear envelope is a double-layered plasma membrane like the cell membrane, although without membrane proteins.
answer:
The nuclear envelope is a double-layered plasma membrane like the cell membrane, although without what?


question:
Exotic species are also called invasive species because they threaten other species.
answer:
Why are exotic species also called invasive species?