Which Is NOT a Possible Explanation for Gender Imbalance in a Population

Which is NOT a possible explanation for gender imbalance in a population pyramid?

A) Females are genetically more likely to occur.
B) In most wars, more males are killed than females.
C) In developed countries, females live 5 years longer than males.
D) In countries with a cultural preference for males, females do not receive sufficient nutrition or health care.