The sum of 3rd and 15^{th} elements of an arithmetic progression is equal to the sum of 6th, 11th and 13^{th} elements of the same progression. Then which element of the series should necessarily be equal to zero?

1st

9th

12th

None of the above.

Correct Answer : C

855,7695,69255,623295,_____

123, 211,299,156,244,____

There are 5 people - A, B, C, D, E, standing in a queue. How many ways are available to form the queue such that D is not ahead of E ?