TO FIGURE OUT how and when ancient fish first CRAWLED from the ocean onto land, Neil Shubin is about to head to the mountains of Antarctica. Leaving behind family and friends for the upcoming holidays, __________ and a team of five other scientists and a mountain guide will be camping at the base of a remote mountain range that was a tropical river delta around 385 million years ago.

Using ropes and climbing gear, the scientists will be scouring cliffs and ridge lines for fossil deposits containing the earliest tetrapods, or four-limbed animals. This is the latest expedition on a fossil-hunting career that has taken Shubin to places as disparate as Ethiopia, West Virginia, and the two poles. To understand better the development of fish into land animals, __________ has to locate fossils that tell __________ the speed and direction of evolution.__________

We are looking to find the intermediates between water and land,‖ says Shubin, a vertebrate paleontologist at the University of Chicago. ―To do that, we need to find rocks that are the right type, the right age and that are exposed.‖

The fossils __________ hopes to find are preserved in sedimentary deposits that exist in several places around the globe, but what makes Antarctica special for fossil hunters is the fact that there is no rain or snow to erode THEM away.

After the fish DIED those millions of years ago, THEY were covered by layers of silt and mud in the river delta. Over time, that mud TURNED to rock, and the bones BECAME fossilized. Now the fossils are part of the Transantarctic Mountains, a 2,000-mile spiny ridge that divides East and West Antarctica and is dotted with deep crevasses and treacherous glaciers. The same dry, cold conditions that make life impossible for almost all current life forms (except microscopic algae or bacteria) in these mountains help preserve fossils for millions of years.

(By Erick Niller, adapted)

Which option best relates to the verb in the past tense in lines 2, 30 and 31?