According to the article Gimme the Ball concerning the development of object permanence, the apparatus in the main study by Goubet & Clifton which was used by the side-experienced group most resembled an upside down version of the letter:

While most U.S. births take place in a hospital, about ______ percent occur in freestanding birthing centers

Fill in the blank with correct word

Groups of people with whom one compares oneself are called

a) associates. b) cohorts. c) reference groups. d) social reference.