Which of the following is true of dead zones? a. They are formed when a volcanic eruption covers the soil with ash. b. They are caused by an increase in the saltiness of the land, making the land infertile. c. They are formed when a specific area lacks nutrients and vitamins. d. They are formed when excess oxygen accumulates in a specific region. e. They are caused by algae blooms that occur when agricultural fertilizers and waste runoff enter natural waterways