Options:
[I] mercury.
[II] pluto.
[III] uranus.
[IV] mars.
Q: Answer the following question given this paragraph: Mars is the easiest planet to observe. As a result, it has been studied more than any other planet besides Earth. People can stand on Earth and observe the planet through a telescope. We have also sent many space probes to Mars. A car-sized robotic rover, Curiosity, arrived on the Red Planet in August 2012. Curiosity joins Opportunity, which has been active since 2004. Q: What planet is the easiest to observe? A:
A:
[IV]