Established in 1988, Chengdu Hi-tech Industrial Development Zone (Chinese: 成都高新技术产业开发区; pinyin: Chéngdū Gāoxīn Jìshù Chǎnyè Kāifā Qū) was approved as one of the first national hi-tech development zones in 1991. When was the Chengdu Hi-tech Industrial Development Zone approved?