What does the word “field” mean in the sentence “Some people are worried that electromagnetic fields from electric power lines could increase the risk of cancer.”?

A、 an area of grass, for example in a park or on a farm ;
B、an area of land or sea bed under which large amounts of a particular mineral have been found;
C、an area in which that particular force is strong enough to have an effect;
D、a particular subject of study or type of activity