[問題]賴水信高分文法P337 Q1 (已收錄)

[elizabeth88]

Anthropologists having found that the phonology and syntax of the native american Indian languages are even better developed and more complicated than our civilized society can imagine.
答案是b,改成have found,但紅色字部份,that後面為什麼可接主詞加動詞1是are後又接一個動詞2是imagine呢,請問這是什麼句型?