Subject: mathqa
Question: a certain junior class has 100 students and a certain senior class has 80 students . among these students , there are 6 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 will be a sibling pair ?
Option: 
A: 3 / 40000
B: 9 / 2000
C: 3 / 4000
D: 1 / 3600
Answer: C