Patient participation in free cataract surgery: a cross-sectional study of the low-income elderly in urban China oleh Bo Qu, Hui Chen, Xuhua Tan, Xiaohang Wu, Haotian Lin, Zhenzhen Liu, Lixia Luo, Bo Zhang, Erping Long, Duoru Lin, Weirong Chen, Yizhi Liu, Haofeng Jiang, Jingjing Chen, Zhuoling Lin, Xiaoyan Li, Jinzhu Tang, Yingfen Lin