The overall population of a species decreased. what could have occurred to make this happen?
the immigration rate increased, while the death rate decreased.
the birth rate decreased, while the death rate increased.
the carrying capacity increased, while the immigration increased.
the emigration increased, while the birth rate increased.

The correct answer is -

The birth rate decreased, while the death rate increased.

Explanation:

Growth of population primarily depends on the factors-

birth rate, death rate, immigration rate, and emigration rate

Population change or population growth can be calculated by using the following equation-

r = (b-d) + (i-e)

Where, r= growth rate

b= birth rate (number of births per thousand of population in an year)

d= death rate (number of deaths per thousand of population in an year).

i= immigration rate (people coming in the area)

e= emigration rate (people exiting/leaving from the area).

Thus, if the birth rate decreased, while the death rate increased, it will lead to overall decrease in the population of a species.

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.