Vygotsky's Zone of Proximal Development (ZPD) Has a Lower Limit

Vygotsky's zone of proximal development (ZPD) has a lower limit and an upper limit. Which of the following best describes these limits?

A) The lower level is the level of skill reached by the child working independently and the upper limit is the level of responsibility the child can accept with assistance.
B) The lower level is the level of skill reached by the majority of students in a class and the upper limit is the level of skill reached by the top five percent of students.
C) A student's ZPD limits and "readiness to learn" is determined by the results of standardized testing.
D) Teaching should begin at the lower limit of the ZPD and gradually progress toward the upper limit so that the student experiences success.