一道英语语法题 such a mass migration of people from countryside to

1.At no time in histroy__such a mass migration of people from countryside to city as is happening now.
这个用倒装
B has there been D has there
这两个中该用哪个?两个区别是什么啊?
1.___were the restrictions that prevented blacks voting.
A.Gone B Going C Go D Having gone
___were the restrictions
A,D的区别是什么?
1.At no time in history has there been such a mass migration of people from countryside to city as is happening now.（当下,如此大规模的人口由农村向城市迁移,在历史上绝无仅有）.
可以将倒装句进行还原,There has been a mass migration mass migration of people from countryside to city as is happening now at no time.has there 没有任何意思的,has there been 中的 has been 是表示现在完成时,与 there be 放在一起使用.
2.Gone were the restrictions that prevented blacks voting.（阻碍黑人进行选举的限制因素已经消除了.）
调整为 The restrictions that prevented blacks voting were gone.
Gone 是消失、失去的意思.Gone with the wind.（随风而逝）.
Were gone 表示的是被动语态；Having gone 不表示任何意思,语法上错误