Which of the following is the best definition of public goods"?
a
none of these are good definitions
b
things (or actions) that are good for the group, but that are
inconvenient, expensive, or even dangerous for an individual to
provide
c
goods and services available to the public at very low cost
d
whatever is good for the group as a whole.

D. what is good for the group as a whole

Explanation:

There are many variations of the definition of public goods, but this, I think, is the best answer. Even though, there is actually no "best" definition.

i'm thinking (a) to keep sand in place; ////////////

explanation: a sand fence or sandbreak, like a snow fence, is used to force windblown, drifting sand to accumulate in a desired place. sand fences are employed to control erosion, sand dune stabilization, keep sand off roadways, and to recruit new material in desert areas.

the correct answer is:

a population changes in response to a change in the environment.