Some research supports the idea that approximately 50 of our happiness is accounted for by factors ...

Some research supports the idea that approximately 50 of our happiness is accounted for by factors that are relatively stable and difficult to change. Which of the following is used to support this view?
 
  A) Identical twins are more similar in their level of happiness than fraternal twins.
  B) Identical twins are less similar in their level of happiness than fraternal twins.
  C) Being kind to others can increase our levels of happiness.
  D) Counting our blessing can increase happiness.
  E) Regular exercise increases happiness.