Multiple penetrations of spermatozoa into one oocyte are impossible. Why? What is it due to?

Multiple penetrations of spermatozoa into one oocyte are impossible. Why? What is it due to?

1.  Acrosomal reaction


2.  Oocyte completes meiosis


3.  Spermatozoon penetration of oolemma


4.  Cortical reaction


5.  *Appearance of tunic of fertilization