What does it mean to define "boundaries" of an ecosystem?

Which type of neuroglia is found outside of the brain?

What causes darkening of the skin as melanin production increases

The carbon cycle is best defined as a process in which a. carbon changes from inorganic forms to organic forms and back b. carbon is changed into other elements such as oxygen or nitrogen c. carbon is continually created from the sun’s energy by plants d. carbon is consumed and regenerated from other elements such as oxygen and nitrogen