Subject: mathqa
Question: a certain junior class has 1,000 students and a certain senior class has 800 students . among these students , there are 20 siblings pairs , each consisting of 1 junior and 1 senior . if 1 student is to be selected at random from each class , what is the probability that the 2 students selected at will be a sibling pair ?
Option: 
A: 1 / 40,000
B: 3 / 40,000
C: 1 / 15
D: 1 / 3,600
Answer: A