Which of the following is an example of a density-independent factor limiting population growth?

a. competition for food
b. the presence of predators
c. a forest fire that was caused by lightning
d. an outbreak of wasting disease in elk

The best answer to this question is:

C. A forest fire that was caused by lightning.

A forest fire could kill for example all the individuals of a certain species in a given area - this would be independent of the population density of the species. All the other factors would affect the population the more, the more dense it is.
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I'm pretty sure there's a geometry rule for   anyways, if the triangle is equilateral, and the circle is circumscribed, the circle is outside of the triangle.  to find the area, you need the radius.  the radius should be a straight line from the center of the triangle to any of it's points.  draw lines from the center of the circle to each of the triangle points.  draw three more lines from the center to the middle of the sides of the triangle.  you should now have six separate 30-60-90 triangles inside of your original triangle. use your 30-60-90 rules to solve for the radius, then the circumference. 

B. to study the spatial patterning of forests and forests fires and plan for better use of fire fighting resources in the u.s.