【单选题】

Questions 17-20 are based on the following passage. You now have 20 seconds to read Questions 17—20. Why is it difficult for adoptees to find out about their birth parents

A、They are usually adopted from distant places.
B、Their birth information is usually kept secret.
C、Their birth parents often try to conceal their birth information.
D、Their adoptive parents don’t want them to know their birth parents.