First satellite was Sputnik and it was a race then to send satellites!! Everyone starts experimenting how to launch.
first try was on a dog , Laika !! It was send to space as it is long after sputnik is placed in orbit !!
Now you know everyone knows rocket science and the most active are ISRO (India) and NASA (USA) !!
Its actually C. I did the question on USA test prep and it said the correct answer was C.
The distance between two particles that are <em>in phase</em>