Must be true questions (or Inference questions) test our ability to find an answer that has direct support from the statements provided. When tackling these questions, it's important to keep two things in mind:

1) Work slowly through the statements, considering how each relates logically to subsequent statements.

2) Incorrect answers very often seem like they could be true, but the mere possibility of validity is insufficient on the GMAT.

As far as the Rienzi is concerned we know the following:
It sank fast (cause unknown).
Normally when a ship sinks that fast it will not fully flood (one way to unsure full flood is sabotage).
A ship with incomplete flooding, if it sinks deep, will implode.
The Rienzi fully floded; it sank deep and did not implode (based on deep-sea photos).

Based on these statements only C is a valid inference. We don't know if sabotage was involved, but we do know if the Rienzi was a normal, fast sinking ship that sank deep, it would not have fully flooded and thus imploded. So, it is unusual (read: not normal) that it reached the ocean floor intact (unless of course sabotage was to blame).

(A) could be true, but we know nothing of the Rienzi's construction.

(B) cannot be true, because a ship that wasn't fully flooded by the time it reached the ocean floor would have imploded.

(D) could be true, but we know very little about how rate of sinking relates to implosion. Sinking more slowly doesn't necessarily mean that it sank slowly (just more slowly relative to the already fast rate).

(E) cannot be true. The statements tell us that any ship that is not full flooded will implode. You cannot argue with this evidence on test day. Take the statements given to you on the GMAT as true (at least to the author).

Hope that helps.

Thanks Brad

Clear Explanation...Cheers