In this task, find the most appropriate number to replace the blank (indicated with _ ) and express it in words.

[Q]: Cigarette smokers have up to a _ times higher incidence of bladder cancer than non-smokers.
[A]: four

[Q]: Retinoic acid regulates oral epithelial differentiation by _ mechanisms.
[A]: two

[Q]: Scarcity occurs when, at a _ price, quantity demanded is greater than quantity supplied.
[A]:
zero