Geologists believe that the Bering land bridge, over which human being

harikrushna Kotadiya One way to think of this is that geologists are describing our current beliefs, so "are" is more appropriate than "were." From there, we can eliminate D because we're not just say "we think of them this way." We actually think that this happened, and it happened in the past.

However, all the "were" choices are wrong for other reasons, too. Both B and C seem to be applying "first" to "thought." This would mean that this is what we thought at first, when what we want to say is that we think this is where people entered first. Also, all three (B, C, E) seem to place "enter" and "thought" at the same time. In other words, people thought that humans were currently entering the Americas over the land bridge! This doesn't make sense.

The complex phrase "are thought to have entered" in A is needed to convey the complex relationship of events. It means we currently think something. What do we think? That humans first entered over the Bering Land Bridge before it disappeared 14,000 years ago. We could avoid some of this awkwardness if we had a subject--in other words, if we specified who was doing the thinking! Then we could say something like "Scientists think that humans first entered the Americas over the Bering Land Bridge, which they believe disappeared 14,000 years ago."