The ocean bottom --- a region nearly 2. 5 times greater than the total land area of the Earth --- is a vast frontier that even today is largely unexplored and uncharted, until about a century ago, the deep-ocean floor was completely inaccessible, hidden beneath waters averaging over 3,600 meters deep. The word "inaccessible" is closest in meaning to which of following? (A) unrecognizable (B) unreachable (C) unusable (D) unsafe