短文改错

     Great changes have been taken place in my village．Ten years ago, the small and low houses, which lie

on the north of a lake, produced much waste．As the result, the lake was serious polluted．To the west

was a sandy field, that was the source of dust．Better still, a brickyard on the southwest side of the lake

used up a great deal good soil．Now the poor house have been replaced by green trees and the brickyard

by tall buildings．The sandy field that the villagers used to ploughing has been covered with fruit trees．

On the lake the villagers often spend his spare time boating and fishing．

改错:                                                                                                                                                      

一个登月的宇航员，能用一个弹簧秤和一个质量为m的砝码，估测出月球的质量和密度吗？写出表达式（已知月球半径R和引力常量G）。