一道SAT语法题During the labor dispute,barrels of potatoes were em

一道SAT语法题
During the labor dispute,barrels of potatoes were emptied across the highway,and they thereby blocked it to all traffic.（P599.4）
(A) highway,and they thereby blocked it to all traffic
(B) highway and thereby blocking it to all traffic
(C) highway,by which all traffic was therefore blocked
(D) highway,and therefore this had all traffic blocked
(E) highway,thereby blocking all traffic
C我倒是明白了,SAT里which不可指代句子,那ABD为什么能立即排除呢,是因为指代不清吗?我是菜鸟,才开始准备SAT,各种不理解~……