Can you explain why this answer is correct (LSAT study ?)?

Question

For some reason I'm just not getting this one question. I tried to have someone explain it to me but it's just not making sense for some reason. The link is:
http://www.getprepped.com/Get-Prepped-Sample-LSAT.pdf
And it is Question #7 in Section 2 (about ship in Atlantic Ocean). The answers & explanations are at the bottom of the test. Will someone tell me a quick explanation of why that answer is correct? (I keep getting hung up on the ship sinking around 850 not hundreds of years after that.)

Answer 1

You are looking for something which breaks the conventional wisdom that any wrecks in the sedimentary layer could NOT have been ships which sailed earlier than 900 AD., even holding all of our scientific knowledge true. I tend to solve these by process of elimination.

A. This actually is the tricky answer. Even if it is determined that some of the ships in the sedimentary layer were built before 900 AD, that does not mean that this is when they actually sailed. Thus, those ships were probably ships that sank when they were old, after 900 AD. The trick is the difference between when the ship was built and when it sailed.

B. If this were true, the ship would have sunk too early to be in the sedimentary layer.

C. This branch is unrelated to the question, talking about the carbon-dating of fish, not ships.

D. This is the correct answer. It would be possible for the ship to have sunken in 850 AD. if it did not immediately sink into the sedimentary layer and get covered up. If it is true that some ships only fell into the sedimentary layer 100 or more years after they sank, then the date of sinking is not the relevant one, what is relevant is the date at which it fell into the sedimentary layer. If this was AFTER 900 AD, then it is possible that this could be the Viking ship they are talking about.

E. This one actually reinforces the original reason they said it could NOT be that ship, but implying that it sank before it reached its destination, in 850 AD. If that were the case, and the information given in part D were not the case, it would have sunken too early to be in the sedimentary layer.