Which of the Following Represents an Alarming Positive Feedback Loop

Which of the following represents an alarming positive feedback loop of global warming?

A) Increasing temperatures raise humidity,which further increases temperatures.
B) Decreased pH of the ocean increases the rate at which carbon dioxide is absorbed by the oceans from the atmosphere.
C) Increased use of fossil fuels adds ozone to the stratosphere,which traps more heat.
D) Increased levels of atmospheric carbon dioxide decreases photosynthesis,which further increases carbon dioxide atmospheric levels.