After a shaky start, the Martian flotilla that has arrived over the past few weeks is getting down to business. Two of the five craft in it seem to be working perfectly. Two are lost, And a fifth is sick, but undergoing treatment. The most spectacular pictures so far have been provided by Mars Empress, the European Space Agency’s contribution to the fleet. On January 28th this reached its final working orbit (which takes it over both poles, and thus allows it to see the whole of Mars over the course of a few days as the planet revolves beneath it). It has, however, been sending back data since shortly after it arrived, and a few days ago its controllers released a series of beautiful photographs, including a stereo image of Valles Marineris, a huge canyon that may have been formed by flowing water. The most scientifically significant result, though, has come from Opportunity, America’s second Mars rover. One of Opportunity’s cameras has photographed evidence of stratification. in nearby rocks. Such stratification indicates that the rocks concerned are sedimentary. The layers could be repeated wind-blown deposits, or consist of ash from successive volcanic eruptions. But the terrestrial rocks they most resemble are ones that have formed under water. The reason everyone is getting so excited is Because there is a widespread assumption that any form of tire which might dwell on Mars would need liquid water to live--or, even if it could now subsist by extracting moisture from ice, would have needed liquid water to evolve to that stage. Mars has seen more probes launched towards it than all of the other planets put together precisely because of this hope that it might harbour life. So there is a lot riding on the answer--not least the funding of future missions. Besides its scientific significance, the success of Opportunity has also helped to distract attention from the sudden refusal of Spirit, the first American rover to arrive on Mars, to talk to its controllers. This craft had tentatively, but successfully, nosed its way off its landing platform, and was about to drill its way into a nearby rock prior to doing a spot of chemical analysis, when it went silent. However, the engineers at NASA, America’s space agency, are nothing if not resourceful, and they have a good record of carrying out running repairs on spacecraft that are millions of kilometres away. In the case of Spirit, they think that one of the craft’s memory chips has got cluttered up with files created On the journey to Mars. That caused another chip, which manages the first, to throw a wobbly and to keep rebooting the computer. They are currently testing this idea by loading a diagnostic program on to the computer. In addition, as a precaution, they have deleted excess files from the equivalent memory chip on Opportunity. Spirit’s spirits may thus revive. As to the failures, the Japanese abandoned their fly-by craft Nozomi in December, and the British team in charge of Beagle 2, which is presumed to have landed on December 25th but from which no signal has been received, also seems to have called it quits. Still, a 40～60% success rate (depending on whether Spirit is brought back into commission) is not bad by the historical standards of missions to Mars. Now, the real science begins. What does the word "they" (Par

A. 3, Line 5) refer toA. "the layers"B. "wind-blow]a deposits"C. "volcanic eruptions"D. "the terrestrial rocks"