单项选择题

Questions 17~20 are based on the following talk about adoption of a child. You now have 20 seconds to read Questions 17~20. Why is it difficult for adoptees to find out their birth parents

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