题目

At, call, try, discover, however, beyond, travel, send, be, understand, reach， goodScientists think that there has    1    life on the earth for millions of years.    2   , we haven’t found life on other planets yet. Why? The earth is a planet and it goes around the sun. There are other planets that also go around the sun. The sun and its planets    3    the solar system(太阳系). The solar system is a small part of our galaxy(银河系). The stars we see at night are the suns in other solar systems. There are more than 200 billion stars in our galaxy, the MilkyM为何整数时，9m2+5m+26能分解成两个连续自然数之积．