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in their present stage of devel opment they cannot lift massive telescopes，keep them steady while the observations are being carried out，and then return them safely． Balloons are much easier to handle，and are also vastly cheaper．Their main limitation is that they are incapable of rising to the ionosphere．A height of between 80，000and 90，000 feet is as much as can reasonably be expected，and SO ball)on borne instruments can contribute little to either ultraviolet(紫外线)astronomy or X -ray astronomy．All the same，the balloon has much to be said in its favour，since it can at least carry heavy equipment above most of the atmospheric mass thus eliminating blurring and unsteadiness of the images． Moreover，water vapour and carbon dioxide in the lower air absorb most of the inflared(红外线)radiations sent to us from the planets．Balloon ascents overcome this hazard with ease． A.Rockets and artificial satellites can go higher than balloons．B.Rockets and artificial satellites are difficult to handle．C.Balloons have limitations．D.Balloons have advantages over rockets and satellites in observation 参考答案： D Balloons have advantages over rockets and satellites in observation----------------------------------------1、I took lots of vitamin C，however，I stilI came down with the flu 参考答案： 1 took lots of vitamin C．However，I still came down with the flu． 1 took lots of vitamin C．However，I still came down with the flu．----------------------------------------2、I never saw SO many cheerful，courteous students students and helpful． 参考答案： I never saw so many cheerful，courteous and helpful students． I never saw so many cheerful，courteous and helpful students．----------------------------------------3、He wanted either tO buy a coat or buy a skirt for his
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