Becoming an expert in a field requires thousands of hours of practice

In his decades-long study of giftedness, Lewis Terman found

a. that there were surprisingly few professionals in his sample. b. there were a great many professionals and even an "Einstein" or two. c. that most gifted children generally grew up to be healthy and happy adults. d. academic success was unrelated to IQ. e. that high IQ was practically a guarantee of wealth and fame.

Dr. Clinton measures children's intelligence ahead of time and then assigns an equal number of high- and low-scoring children to each treatment condition. Dr. Clinton is using

A) a confounding variable. B) matching. C) a natural, or quasi-, experiment. D) random assignment.