All of the following are evidence that a chemical reaction has occurred except
a. color change
b. formation of a precipitate
c. bubbles of gas
d. all of the above are evidence of a chemical reaction

The greenhouse effect a. something that has occurred for millions of years b. an unnatural phenomenon caused by humans that maintains earths temperature range. c. the results of the differences in the angle of the suns rays d. an unnatural phenomenon that causes heat energy to be radiated back into the atmosphere