There are d cities. What is the number of airlines that connected 2 different cities (airline from city A to city B is different with airline from city B to city A)?  Choose the correct answer:
A) n(n-1)	B) n(n+1)	C) d(d-1)	D) n(n+1)/2	E) n2
First of all, the possible number of airlines is d. Then, the possible number of airlines departing from one city to another becomes (d-1). Hence, the answer is d(d-1), d(d-1)
<result>C</result>
There are d cities. What is the number of airlines that connected 2 different cities (airline from city A to city B is different with airline from city B to city A)?