21.   Over five thousand years ago, archaeologists have discovered that Mesopotamian cooks were preparing chicken with coriander and tamarind, just as Egyptian cooks are still doing today.

(A) Over five thousand years ago, archaeologists have discovered that

(B) Over five thousand years ago, archaeologists had discovered that

(C) Archaeologists have discovered that, over five thousand years ago,

(D) Over five thousand years ago, as archaeologists have discovered it,（C）

(E) Archaeologists have discovered, over five thousand years ago, that

为什么是C,不是A?

我觉得如果这题放在白勇书上,肯定解释强调的重点变了,A更好

求高人解