In habituation-dishabituation studies examining infant cognitive abilities,how did researchers present the addition or subtraction "problems" to the infants

A)  handing the infant objects one at a time 
B)  by showing the infants groups of objects that are covered and uncovered by a screen 
C)  by showing the infants objects on a table-top 
D)  using mathematical statements of the problem (for example "5 + 5 = 10")

Researchers typically use a habituation-dishabituation paradigm in which they show the infants a repeated stimulus until they become habituated, meaning they are no longer interested in the stimulus. Then, they present a novel or changed stimulus (such as an extra object or the removal of an object) to see if the infant will show renewed interest, indicating that they have noticed the change or can perform basic addition or subtraction. The most common way to present these changes is to use a screen that covers and uncovers groups of objects. Handing objects one at a time (A) or showing them on a table-top (C) may be too simplistic and not allow for the precise control of variables necessary for rigorous scientific study. Using mathematical statements (D) would likely be too advanced for infants who have not been exposed to math education yet.