We define that K@J is the product of j number from k in increasing order for positive integers K, J. For example, 6@4=6*7*8*9. If A=2020and B=2120, what is the value R of the ratio A/B?
Pick one:  A) 1 / 2  B) 1 / 3  C) 2 / 3  D) 1 / 4  E) 1 / 5

Rationale: "r - > a / b = 20 * 21 * … … * 39 / 21 * 22 * … . * 39 * 40 = 20 / 40 = 1 / 2 . therefore , the answer is a ."
Answer: A