How strange it was to see an alien in the street!

这句子原句型应该是
it was strange to see an alien in the street!
这种 it is + adj + to do sth.的常用句型
将它变为感叹句型就是：
how + adj + it is + to do sth.（可以对比与上面句子的变化）
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如果变成用what 来变感叹句,则what 后面+名词短语
题目中,就变成 what a strange thing it was to see an alien in the street!