As matter and energy move from grasses to coyotes the amount of available
a. always decreases and population size always increases.
b. always increases and population size always decreases
c. always decreases but population size may increase or decrease
d. increases or decreases but population size remains the same

C. Always decreases but population size may increase or decrease.

According to the 10% law in ecological dynamics, the energy decreases by 10% when organism in lower trophic level being consumed by organism in the higher trophic level. The highest trophic level receives the lowest energy.

As matter and energy move from grasses (lower trophic level) to coyotes (higher trophic level) the amount of available energy will always decrease as per the 10% law but population size may increase or decrease.  

any abiotic or biotic factor in an ecosystem that causes a population's size to slow or decrease is a limiting factor.

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