If each participant of a chess tournament plays exactly one game with each of the remaining participants, then 210 games will be played during the tournament. What is the number of participants?  Choose the correct answer.
A) 18  B) 19  C) 20  D) 21  E) 22

Rationale: "let n be the number of participants . the number of games is nc 2 = n * ( n - 1 ) / 2 = 210 n * ( n - 1 ) = 420 = 21 * 20 ( trial and error ) the answer is d ."
Answer: D