Which of the following statements is true? choose one:

a. most climate scientists believe that current temperature increases are due entirely to natural cycles of short-term climate changes.
b. the worst-case scenario predicts that if global warming continues at its present rate, by 2050 the average annual temperature will have increased from 15°c to 20°c.
c. the intergovernmental panel on climate change (ipcc) has ruled that all future research use the one computer climate model that it has concluded is the best.
d. carbon dioxide concentration of the atmosphere has increased in the last century.