The community knew the effects of Chemical X and Chemical Y when used independently of one another.Safe limits were set for the use of each chemical.When both chemicals were released into the environment at their safe levels,but at the same time and location,massive quantities of fish died.What is the most probable explanation for this occurrence? 

A)  negative feedback 
B)  synergistic interaction 
C)  positive feedback 
D)  homeostasis

MOST people in the United States are ____________________,which makes the country rely heavily on Saudi Arabia.

What happens to material that is hotter than its surrounding material deep within the Earth?

A) The hot material slowly moves up toward the surface.
B) The hot material moves slowly down toward the core.
C) Nothing. The hot material doesn't move at all.

Which of the following is NOT among the disadvantages of using solar energy to generate high-temperature heat and electricity? 

A)  It requires a backup storage system. 
B)  It is expensive. 
C)  It requires high land use. 
D)  It releases carbon dioxide emissions.