We cannot determine, whether the number of automobiles weighing more than 3500 pounds is greater or smaller than the number of cars that weigh less than 3500 pounds, because we don't know anything about the 18 weighs.

e.g. 4 cars weigh each 15,000 pounds (sum of these 4 cars = 60,000). Then the remaining 14 cars would have to weigh averagely 3000/14 pounds (18 *3,500 = 63,000 ; 63,000 - 60,000 = 3,000)
--> B > A

it is also possible that the two quantities are equal:
--> 9 cars weigh each 3400 pounds and 9 cars weigh each 3600 pounds
--> A = B

Since it is possible that B> A and B=A (and A>B), the relationship cannot be determined from the information given, hence D