What is the definition of Concentration Gradient?

Would really appreciate it, i have an idea but not completely sure

(99 points) be serious! how do farts work? how do you fart without it stinking? serious answers only

Rarefaction is used for which of the following? a. creating a representative sample b. studying how disease impacts populations c. rediscovering species thought to be instinct d. determining which sea creatures live in an area