The purpose of the Lewis and Clark expedition was ______

Câu hỏi: Reading 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 question:
After the United States purchased Louisiana from France and made it their newest territory in 1803, President Thomas Jefferson called for an expedition to investigate the land the United States had bought for 15 million USD. Jefferson's secretary, Meriwether Lewis, a woodsman and a hunter from childhood, persuaded the president to let him lead this expedition. Lewis recruited Army officer William Clark to be his co-commander. The Lewis and Clark expedition led the two young explorers to discover a new natural wealth of variety and abundance about which they would return to tell the world.
When Lewis and Clark departed from St. Louis in 1804, they had twenty-nine in their party, including a few Frenchmen and several men from Kentucky who were well-known frontiersmen. Along the way, they picked up an interpreter named Toussant Charbonneau and his Native American wife, Sacajawea, the Shoshoni "Bird Woman" who aided them as guide and peacemaker and later became an American legend.
The expedition followed the Missouri River to its source, made a long portage overland though the Rocky Mountains, and descended the Columbia River to the Pacific Ocean. On the journey, theyencountered peaceful Otos, whom they befriended, and hostile Teton Sioux, who demanded tribute from all traders. They also met Shoshoni, who welcomed their little sister Sacajawea, who had been abducted as a child by the Mandans. They discovered a paradise full of giant buffalo herds and elk and antelope so innocent of human contact that they tamely approached the men.
The explorers also found a hell blighted by mosquitoes and winters harsher than anyone could reasonably hope to survive. They became desperately lost, then found their way again. Lewis and Clark kept detailed journals of the expedition, cataloging a dazzling array of new plants and animals, and even unearthing the bones of a forty-five-foot dinosaur.
When the party returned to St. Louis in 1806 after travelling almost 8,000 miles, they were eagerly greeted and grandly entertained. Their glowing descriptions of this vast new West provided a boon to the westward migration now becoming a permanent part of American life. The journals written by Lewis and Clark are still widely read today.
The purpose of the Lewis and Clark expedition was ______
A. to purchase land from France.
B. to establish trade with the Otos and Teton Sioux.
C. to find the source of the Missouri River.
D. to explore territory purchased by the United States.