A、Solar activity. B、Greenhouse effects. C、Warming on all the planets and their moons. D、No definite answer. D推理题。对于前三个选项，文章中没有肯定

There have been claims that warming on Mars and Pluto are proof that the recent warming on Earth is caused by an increase in a solar activity, and not by greenhouse gases. But we can say with certainty that, even if Mars, Pluto or any other planets have warmed in recent years, it is not due to changes in solar activity.
    The Sun’ s energy output has not increased since direct measurements began in 1978(see Climate myth special: Global warming is down to the Sun, not humans). If increased solar output really was responsible, we should be seeing warming on all the planets and their rooms, not just Mars and Pluto.
    Our solar system has eight planets, three dwarf planets and quite a few moons with at least a rudimentary atmosphere, and thus a climate of sorts. Their climates will be affected by local factors such as orbital variations, changes in reflectance(albedo)and even volcanic eruptions, so it would not be surprising if several planets and moons turn out to be warming at any one time.
    However, given that a year on Mars is nearly two Earth years long, and that a year in Pluto lasts for 248 Earth years. It is rather early to start drawing conclusions about long-term climate trends on the outer bodies of the Solar System.
    Images of Mars suggest that between 1999 and 2005, some of the frozen carbon dioxide that covers the south polar region turned into gas(sublimated). This may be the result of the whole planet warming(see Mars images hint at recent climate swings).
Questions 16 to 18 are based on the passage you have just heard.
16. What is the most possible reason for the warming of Mars and Pluto?
17. Which of the following statements about Pluto is correct?
18. What is the author’s attitude towards the climatic changes of Mars and Pluto?