Pls read follow article and answer one question. Thanks.

Bai, Ying, and James Kung. 2015. "Diffusing Knowledge While Spreading God's Message: Protestantism and Economic Prosperity in China, 1840-1920." Journal of the European Economic Association 13 (4): 669-698.

Readings' PDF Link (P2-P26): http://www.sss.tsinghua.edu.cn/__local/7/81/61/06E829E1B608B2D540721640216_2F9420A6_1A11E5.pdf?e=.pdf

Question: Which of the control variables do you consider most important in testing the effect of Protestantism on economic prosperity in China. How do the authors prove that culture is not the channel through which the significant effect of Protestantism had on economic prosperity in early 20th-century China?