2. Which of the following plays a key role in body growth according to the text?
C
[解析] 第三段的第三句话:“But as diet and health improved, children and adolescents have, on average, increased in height by about an inch and a half every 20 years”。此句说明身高增长的因素包括饮食和健康的提升。饮食和健康的提升可以综合为生活水平的提高。这符合选项C的意思。选项A的意思是:基因变化。选项B的意思是:自然环境。选举D的意思是:日常锻炼。这几项原文未提到。故答案为C。