What is the equation to calculate the number of symmetric keys needed, and how many keys are required if 10 people need to communicate using symmetric keys?

What is the equation to calculate the number of symmetric keys needed, and how many keys are required if 10 people need to communicate using symmetric keys?

A.
N(N+1)/3 and 40 keys are needed

B.
N(N-1)/2 and 45 keys are needed

C.
N(N/2) -1 and 35 keys are needed

D.
N(N+1) and 25 keys are needed

Explanation:
The correct equation to use to determine how many symmetric keys are
needed is N(N-1)/2. If 10 people needed to communicate via symmetric encryption
processes, then 45 keys are needed.