A is the primary reason. B is true, but it is not so much that women take time off and are therefore paid less, it is that women take time off and miss out on

A is the primary reason. B is true, but it is not so much that women take time off and are therefore paid less, it is that women take time off and miss out on a few rounds of promotions and get behind a bit. You only take 5 years out of a 40 year career, but you might take out two of the total promotions and that adds up to a lot. C happens but rarely in the US is it that a woman makes 77% of a man for exactly the same occupation, the difference might be 5-10% but not 23%.