SAT GRAMMAR 2题 求详解 （ ）内为有可能的错误

SAT GRAMMAR 2题 求详解 （ ）内为有可能的错误
1.During the labor dispute,barrels of potatoes were emptied across the (highway,and they thereby blocked it to all traffic).
A.highway,by which all traffic was therefore blocked
B.highway,thereby blocking all traffic
Q:为什么不能选A,这里的BY WHICH有什么问题么?我觉得可以理解成 all traffic was therefore blocked by那些土豆~为什么B可以呢?
2.In(those cities) (in which) public transportation is adequate,fewer traffic problems occur and pedestrians (are rarely) (involved in) accidents.
Q:ERROR.可是我觉得（in which)有问题啊,因为前面已经有In(those cities)了啊,感觉怪怪的.