Final answer:
The main difference identified is the expense of the missions; Lewis and Clark's expedition was modestly funded, whereas the Mars missions involve significant costs.
Step-by-step explanation:
One difference the writer identifies between the Mars mission and the Lewis and Clark expedition is C) Lewis and Clark's expedition was inexpensive; a Mars mission will be very costly. The Lewis and Clark expedition was commissioned by Thomas Jefferson with a relatively modest budget of $2,500. It was a mission of discovery aiming to map out the Louisiana Territory and seek potential trade routes. In contrast, modern Mars missions, such as the 2011 NASA Curiosity rover, are extremely expensive due to the advanced technology and equipment required for space exploration.