Which sentence is best structured to make a direct statement about one idea?
A. Since NASA was founded, it has sent more than 250 astronauts
into space.
B. NASA is known for sending humans to the moon, but the famous
space program has also sent an unmanned spaceship to Mars.
C. Some people think that NASA should focus on exploring the
universe, but others believe that building a colony on the moon is
the most important goal, even though it will be difficult.
D. The mission of NASA is to explore outer space.