-doing vs -by doing 求牛牛解释

The organic foodindustry has organized a successful grassroots campaign—using Web sites,public meetings, and also mass mailings—that have convinced the Departmentof Agriculture to change the proposed federal regulations for organically grownfood.

A.     using Web sites, publicmeetings, and also mass mailings—that have convinced

B.     using Web sites, publicmeetings, and mass mailings—that has convinced

C.     by using Web sites,public meetings, and mass mailings—in convincing

D.    by the use of Web sites, public meetings, as well as mass mailings—thatconvinced(B)

E.     which used Web sites,public meetings, and mass mailings—in convincing

这道题答案选B，对于这题的答案我没有问题。问题是，在B选项前面加上一个by，会有什么不同的意思吗？还是没有很大影响？
请教牛牛。