Jeff is conducting a literature review on the research on genetics and sexual orientation. He would ...

Jeff is conducting a literature review on the research on genetics and sexual orientation. He would find which of the following statements are supported by research?
 
  a. Most influential genetic studies of sexual orientation involve pairs of non-identical twins.
  b. Only 4 to 5 of the variation in sexual orientation can be explained by genetic influences.
  c. Over the past 30 years, research methods have become more sophisticated, allowing researchers to tease apart the forces of environment and genes.
  d. Scientists have uncovered a gay gene that causes sexual orientation.