SAT语法改错~The rules A( of usage) for B(comas) C(are) a source

SAT语法改错~
The rules A( of usage) for B(comas) C(are) a source of great D(consternation) for many students of writing.E( no error)
书上说是B 因为comma才是逗号.但是C是怎么对滴?不是Rules是主语吗?如果不是,怎么辨认主语滴说,而且C后面是a source滴说~