题目

  A young man, while traveling through a desert, came across a spring of clear water.    1___water was sweet. He filled his leather container so that he could bring some back to an elder  2 __  had been his teacher. After a four-day journey, the young man   3    ( present ) the water to the old man. His teacher took a deep drink, smiled   4   ( warm ), and thanked his student very much for the sweet water. The young man went home   5   a happy heart.      After the student left, the teacher let   6   student taste the water. He spit it out,    __7___(say一个行星，其半径是地球半径的2倍，质量是地球的8倍，则它表面的重力加速度是地球表面重力加速度的：A.16倍B.4倍C.2倍D.0.5倍