Which One of the Following Best Describes a Flux

Which one of the following best describes a flux?

A) Movement of elements or compounds between reservoirs in a biogeochemical cycle
B) A pool or reservoir where an element or compound is stored within a biogeochemical cycle
C) A type of flower used for food and fiber;also known as linseed
D) Variation in output from a large water reservoir,such as output through a hydroelectric dam