Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 30 to 34 

     Though they were not trained naturalists, Meriwether Lewis and William Clark in their explorations of North America in the early nineteenth century came across enough unfamiliar birds, mammals, and reptiles to fill a zoo. In keeping with President Jefferson's orders they took careful note of 122 species and subspecies that were unknown to science and in many cases native only to the West. Clark made sketches of any particularly intriguing creature. He and Lewis also collected animal hides and horns and bird skins with such care that a few of them were still intact nearly two centuries later. While Lewis and Clark failed to meet the mythological monsters reputed to dwelt in the West, they did unearth the bones of a 45 - foot dinosaur. Furthermore, some of the living beasts they did come upon, such as the woolly mountain goat and the grizzly bear, were every bit as odd or as fearsome as any myth. In their collector's enthusiasm, they even floated a prairie dog out of its burrow by pouring in five barrel full of water, then shipped the frisky animal to Jefferson alive and yelping. 

What does the passage mainly discuss?