SAT语法题The Empire State Building,the Sears Tower,the Canadian

楼上的都说的不对,
这里they were build要做 the time的定语从句,而从句时表示时间的,这时候引导词when是不能省略的.因此正解应该是 at the time when they were built.
还有网友说这里应该用单数,其实不然.这里指的是说有上述的建筑物,这些建筑物都算上当然应该用复数.each只对each of theses structures was the tallest in the world at the time 这个句子的谓语生效,对题目中错误之处的定语从句不生效.

they were built 吧

each 开头作代词所以是单数 of 看前面的词语 所以 看each是单数因改为 it was built

it is built