A、Every 2 years. B、Whenever one dies or must resign. C、Every 6 years. D、Every 4 years like the president. C6年一次：... they do not

Senators are people who are elected every 6 years and they represent a whole state. This means there are fewer Senators than Congress Representatives but they do have the power to defeat any bill that has been passed by the Congress. Their function is to provide more security over the decisions and actions of the Congress Representatives, and to make sure that bills, passed by the House of Representatives, are not driven by narrow local interests. In England, the British have an appointed House of Lords to provide a "second sober thought" about new laws. In America, Senators are thought to do the same thing but the primary difference is that they are elected and are very responsive to public opinion. However, because they do not have to face elections for 6 years, it was thought that they may be more rational than populists, although in practice the Senate and the Congress are still very much swayed by the views of public opinion or by powerful lobby groups. If the bill is passed in the Senate, then it goes to the President to either sign or reject. If he signs it, it becomes law, but if he rejects it, that  is to say he vetoes the bill, or refuses to sign it, or as they say, "pocket vetoes it", it will be sent back to the Congress. If the Congress disagrees with the President enough and does not Want to change or amend the bill, they can override the President’s rejection called overriding the veto.