听力原文： We know then that in the US, its the job of Congress to propose new laws, which we call bills, and perhaps to modify these bills and then write on them. But even if the bill is passed in Congress, it still doesnt become a law until the President has a chance to review it, too. And if its not to the Presidents liking, the bill can be vetoed or killed in either of two ways. One is by a veto message. The President has ten days to veto the bill by returning it to Congress, along with the message explaining why its being rejected. This keeps the bill from becoming a law unless overwhelming majorities of both Houses of Congress vote to over-right the Presidents veto. Sometimes they do that. Often, lawmakers simply revise the vetoed bill and pass it again. This time, in the form. the President is less likely to object to, and less likely to want to veto. The other way the President can kill a bill is by pocket veto. Heres what happens. If the President doesnt sign the bill within ten days, and Congress adjourns during that time, then the bill will not become law. Notice that it is only at the end of an entire session of Congress that the pocket veto can be used, not just whenever Congress takes a shorter break, say, for a summer vacation. After a pocket veto, that particular bill is dead. If a lawmaker in Congress wants to push the matter in their next session, theyll have to start all over with a brand-new version of the bill. Questions 16 to 19 are based on the passage you have just heard. 16. What is the main topic of the talk? 17. According to the speaker, what does the veto message explain? 18. According to the speaker, what do lawmakers often do after a veto message is issued? 19. What happens to a bill as a result of a pocket veto?16．