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?

 Choices:

- mercury

- pluto

- uranus

- mars

A:
mars
