Given scientific fact as input, generate the question from this fact such that it can be answered from the input.

Example Input: Compounds with metals and nonmetals sharing electrons are known as ions kinds of compounds.
Example Output: Compounds with metals and nonmetals sharing electrons are known as what kinds of compounds?

Example Input: A fewer predators is most likely to cause the number of rabbits living in an area to increase.
Example Output: Which factor is most likely to cause the number of rabbits living in an area to increase?

Example Input: Resistors connected end to end are said to be in series.
Example Output:
Resistors connected end to end are said to be in what?