Which of the Following Organic Explanations Has Been Offered to Explain

Which of the following organic explanations has been offered to explain the fraternal birth-order effect?

A) The fraternal birth-order effect has no organic explanation. It is simply a trend that has not been shown to have any real evidence or relevance to the discussion of sexual orientation.
B) The Y-chromosome activates a gene more forcefully in each consecutive male birth, which is why this order effect applies to male and not female babies.
C) The rules of biology state that the only way for two boys to be born consecutively is if there is a genetic mutation. This mutation is thought to explain the fraternal birth-order effect.
D) There is a defensive maternal immune response that is triggered by foreign substances produced by each additional male fetus, and this immune response increases with each additional male pregnancy.