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。