Task: Question Generation
Input (English): Background Paragraph: Another obvious change that occurs during puberty is rapid growth. This is called the adolescent growth spurt . In boys, it is controlled by testosterone. The rate of growth usually starts to increase relatively early in puberty. At its peak rate, growth in height is about 10 centimeters (almost 4 inches) per year in the average male. Growth generally remains rapid for several years. Growth and development of muscles occur toward the end of the growth spurt in height. Muscles may continue to develop and gain strength after growth in height is finished. 
Story: Two researches studied adolescents during puberty. Tim studied males, while Dan studied females.
Output (English): Which researcher observed that growth generally remains rapid for several years?