【正确答案】 B

【答案解析】[听力原文]
We define time travel to mean departure from a certain place and time followed (from the traveller's point of view) by arrival at the same place at an earlier (from the sedentary observer's point of view) time. Time travel paradoxes arise from the fact that departure occurs after arrival according to one observer and before arrival according to another. In the terminology of special relativity time travel implies that the time like ordering of events is not invariant. This violates our intuitive notions of causality. However, an intuition is not an infallible guide, so we must be careful. Is time travel really impossible, or is it merely another phenomenon where "impossible" means "nature is weirder than we think?" The answer is more interesting than you might think.
The B-movie image of the intrepid chrononaut climbing into his time machine and watching the clock outside spin backwards while those outside the time machine watch the him revert to callow youth is, according to current theory, impossible. In current theory, the arrow of time flows in only one direction at any particular space. If this were not true, then one could not impose a 4-dimensional coordinate system on space time, and many nasty consequences would result. Nevertheless, there is a scenario which is not ruled out by present knowledge. This usually requires an unusual space time topology (due to wormholes or strings in general relativity) which has not yet been seen, but which may be possible. In this scenario the universe is well behaved in every local region; only by exploring the global properties does one discover time travel.
It is sometimes argued that time travel violates conservation laws. For example, sending mass back in time increases the amount of energy that exists at that time. Doesn't this violate conservation of energy? This argument uses the concept of a global conservation law, whereas relativistically invariant formulations of the equations of physics only imply local conservation. A local conservation law tells us that the amount of stuff inside a small volume changes only when stuff flows in or out through the surface. A global conservation law is derived from this by integrating over all space and assuming that there is no flow in or out at infinity. If this integral cannot be performed, then global conservation does not follow. So, sending mass back in time might be all right, but it implies that something strange is happening. Why shouldn't we be able to do the integral?
The possibility of time travel in GR has been known at least since 1949. The GR space time found by Godel has what are now called "closed timelike curves" (CTCs). A CTC is a world line that a particle or a person can follow which ends at the same space time point (the same position and time) as it started. A solution to GR which contains CTCs cannot have a spacelike embedding—space must have "holes" (as in donut holes, not holes punched in a sheet of paper). A would-be time traveller must go around or through the holes in a clever way.