Researchers (Helle et al., 2004) analyzed rates of twin births in the Sami population of Northern Scandinavia during the eighteenth and nineteenth centuries. They found that (1) a baby born from a singleton pregnancy was more likely to survive to adulthood than a baby born from a twin pregnancy, and (2) the average number of offspring raised to adulthood was higher for women who had twins than for those who never had twins. These data suggest that in this population , human twinning rate was

a. under direction selection

b. under stabilizing selection

c. under disruptive selection

d. under sexual selection

e. undergoing genetic drift