Questions 14 to 16 are based on the following passage. At the end of the passage, you will be given 15 seconds to answer the questions. Now, listen to the passage. What is the most difficult part in the issue of illegal immigration, according to the passage you have just heard
A．Legalizing all immigration, whether it is legal or illegal. B．Dealing with the legal and illegal immigrations respectively. C．Building a long fence along the long border near Mexico. D．Making the legal immigration easier and more common.