Economist Marco Castillo and colleagues showed that nuisance costs--the time and effort people must spend to make donations--reduce charitable giving. Charities can mitigate this effect by compensating donors for nuisance costs, but those costs, though variable, are largely _________ donation size, so charities that compensate donors will likely favor attracting a few large donors over many small donors.

Which choice completes the text with the most logical and precise word or phrase?