阅读短文，下列问题

 B

    A marathon is an endurance (忍耐力) race which covers 26 miles, 385 yards (42.2 km). It is a race for the strong. To run this race, one must have great endurance.

    The history of this tiring race dates back to a war. The war was between the Persian Empire and the Greeks. It was in the year 490 BC. The war took place on a plain (平原) called Marathon.

    The Greeks won. After the war, a man was chosen to announce that the Persians had been defeated (打败) in the battle. The man ran from the battlefield of Marathon to Athens. He ran at top speed without stopping to rest. When he reached Athens, he told the news and died.

    In 1896, the marathon race was included in the first modern Olympic Games in Athens. The race was to remember the runner. It was 24.85 miles (40 km), the distance that the man had been asked to run to give his news.

    All marathons today are a little longer than the one in the first modern Olympic Games. Distance was added at the 1908 O1ympid Games in London, The distance was changed to 26.22 miles to cover the ground from Windsor Castle to White City stadium, with the 2.2 miles added on so the race could finish in front of the Royal Box.

宇宙中存在一些离其他恒星较远的、由质量相等的三颗星组成的三星系统，通常可忽略其他星体对它们的引力作用，已观测到稳定的三星系统存在形式之一是：三颗星位于同一直线上，两颗环绕星围绕中央星在同一半径为R的圆形轨道上运行，设每个星体的质量均为M，则（   ）

方程$\sqrt{1-\left|x\right|}=\sqrt{1-y}$表示（   ）