how many keys would be needed if there were 500 participating users?

In symmetric cryptography, how many keys would be needed if there were 500 participating users?

A.
124,750

B.
249,500

C.
250

D.
250,000

Explanation:
The formula for calculating how many symmetric keys are needed is:
n(n-1)/2.
So, in this example: 500(500-1)/2 = 124,750.