At 6 months old, Elizabeth can distinguish between her mother's and father's voices, which are in different registers: She starts gurgling when she hears her mother's voice and she turns her head when she hears her father's voice. One day, when Elizabeth's father takes her to the grocery store, the woman at the checkout counter speaks to Elizabeth in a voice that is in a similar register to her mother's. In response to the cashier, Elizabeth begins gurgling. Which concept is being illustrated here: gender schemas, gender stereotypes, gender roles, or cognitive development theory? Please explain your choice.