题目

Man has been to the moon. It has been the first step towards future exploration in space. When compared with the distances between Earth and the other planets, the distance between the Moon and Earth is very short indeed. Mars, the nearest planet to Earth is millions of miles away! Traveling to the planets will be man’s next step. Such travels will be more challenging than the trip to the moon and certainly more exciting and dangerous. Recently, two American spaceships, Viking 1 and 2, landed on Mars trying to discover whether that planet has any life on it. So far the signs of life