Critical Reasoning (CR) | Codex Berinensis, a Florentine copy of an ancient Roman

I am still struggling with this quesiton.
Isn't D redundant / does not matter since the premise already confirms that the copyist was mentioning about specific plague that orrcured in 1148?
"Since a letter in handwriting identified as that of the fourth copyist mentions a plague that killed many people in florence in 1148,"

I chose C. what if it takes "more than 1 year" to copy a page? The CB copy would not finish in 1148?

Could someone explain flaws in my logic? Thanks.

---