The most convincing evidence for the importance of adult influence on a child's intelligence comes from a study of "at risk" children. Ramey and Frances Campbell of the University of North Carolina【1】with children born into poverty-line households. The children entered the study by four months【2】age. During the study, one group spent the day in a center where teachers used games and songs to【3】the infants. Another group had no such 4 , but they were given nutritional supplements in【5】 .
During preschool years the children in the early-education group showed IQ advantages of 10 to 20 points. The highest-risk children showed the【6】gains, and at age 15 they had higher reading and math scores.
What【7】for these gains？ Ramey and other scientists say early childhood experiences【8】brain growth.
An infant is born【9】billions of brain cells called neurons. Some are wired to other cells before birth to regulate the【10】of life, such as heartbeat and breathing. Others are waiting to be wired to【11】him or her interpret and respond to the outside world. Experience dictates the hookups. As the child【12】, cells reach out and set up pathways to other cells needed to determine a【13】. For instance, the neurons in the eye send branches to the【14】cortex, which interprets【15】eye sees and, via other branches,【16】the person to react to what is seen. Each time an experience is repeated, the【17】are strengthened.
The first two years of life are an explosion of brain【18】and connections. By age two the【19】has more than 300 trillion connections. At the same time, cells that aren't being connected or used are being【20】.
(1)

A. worked
B. stayed
C. studied
D. talked