Which of the following could be both a natural and a human cause of succession?

Succession in Ecology is the observation of the growth and change in a vegetation community. So the relationship between Succession and Climax in vegetation is that succession is the process of observing the change in a vegetation community from a few pioneering plants to a fully developed, stable community known as a climax community. 

the glands which are associated with each description is given below:

pancreas - is an endocrine and exocrine gland that secretes juices with enzymes for digestion. salivary glands- with protection and digestion in the mouth. liver- secretes bile for digestion.   lachrymal glands - secrete tears to moisten eyes and protect them from particles and infection.   sweat gland -regulates water, salt, and temperature. oil glands - to form a protective barrier on the skin.   goblet cells - secrete mucus for protection.

D. pray lands grasslands swamps