In Reference to Vygotsky's Notion of "Private Speech", Which of the Following

In reference to Vygotsky's notion of "private speech", which of the following is true ?

A) Children develop private speech at birth.
B) When children reach middle childhood, their private speech takes the form of whispering and inaudible muttering.
C) Private speech is only used when solving difficult problems.
D) All of the above alternatives are correct.