题目解析:
题干提问“在9000到4000年间,墨西哥有了什么发展”。
定位第1段最后一句 by about 4,000 years ago, maize cob size had become large enough that people over large areas of the Mexican highlands could subsist mainly on maize.
即 maize cob size become large 为一种 development,could subsist mainly on maize 理解为以玉米 maize 为生,对应选项 D。
故选 D。