Greg Is 10 Months Old, and He Refuses to Crawl

Greg is 10 months old, and he refuses to crawl across the visual cliff even though his mother is encouraging him to do so. When he was 6 months old, he squirmed across it without hesitation. What has changed for Greg?

A) He has matured into understanding that crawling over an edge affords falling.
B) His visual system has matured sufficiently to detect depth.
C) He has developed an insecure attachment with his mother.
D) He must have fallen from an elevated height since his last test on the visual cliff.