Robert's mother pours a cup of milk for him and then realizes that the cup has a small crack in it. not wanting her son to cut his lip, she pours the milk into another cup. the second cup happens to be shorter and wider than the first cup. robert is not upset because he knows the amount of milk has remained the same. robert is at least how old?
a. three years old
b. five years old
c. seven years old
d. nine years old
e. twelve years old