The authors mentioned the interesting correlation between natural red hair and lower pain thresholds compared to persons with other natural hair colors. This is most likely to be due to the fact that. a. genetic factors that impact pain threshold are also somehow related to hair color.
b. natural hair color causes people to have an increased or decreased pain threshold.
c. natural pain thresholds cause people to be born with different natural colors of hair.
d. None of the above was mentioned by the authors as an explanation of the correlation between natural hair color and pain threshold.