In 1905, at the request of the Paris school authorities who wanted a means of identifying children who were slow learners, Binet prepared the forerunner (雏形) of the modem intelligence test. Binet's test consists of 30 questions corresponding in difficulty to the varying abilities of children between the ages of 3 and 13. The questions ranged from simple tasks, such as asking children to identify their eyes, nose, and mouth, to more complex activities, such as constructing a sentence around certain words. The questions were arranged so that a three-year-old child would be expected to answer progressively more difficult ones.
Binet was able to establish standards of performance by first giving the test to a sample composed of children of varying ages. He then carefully recorded their scores and specified norms—the distribution of scores obtained by children at different ages. Thus, a child's intelligence was operationalized(转化为) as the number of items he or she was able to answer correctly. The level achieved indicated the child's mental age (MA). For example, if a child successfully completed all those items typically answered by the average five-year-old and none above that level, the child's MA was said to be five years.
Binet's idea of intelligence measurement was based on his discovery that ______.

A. his daughters walked and thought differently
B. some children were slower learners
C. children's intelligence increased with age
D. intellectual milestones marked each age