Why Can A-Globin Mutations Have an Effect Earlier in Life $\alpha$

Why can a-globin mutations have an effect earlier in life than b-globin mutations?

A) There are fewer $\alpha$ -globin genes
B) The $\beta$ -like globin hemoglobin F (HbF) is produced during fetal development and $\beta$ globin predominates only after birth
C) Fewer $\alpha$ globins are in each hemoglobin molecule
D) More $\alpha$ globins are in each hemoglobin molecule
E) Mutations in the $\alpha$ -globin genes prevent proper developmental expression of other globin genes