单选题
Everyone knows that in the city of Pisa in Italy there is a tower that "leans". The tower is built entirely of white marbles. The walls are 13 feet thick at its base. It has eight storeys and is 179 feet high. There is a stairway of 300 steps built into the walls, which leads to the top. At the top of the tower, you'll see the city and the sea. The sea is six miles away. At the top, the tower is 16.5 feet out of the perpendicular. In other words, it "leans" over by 16.5 feet. If you were to stand at the top and drop a stone to the ground, it would hit 16.5 feet away from the wall at the bottom of the tower! What makes it lean? Nobody really knows the answer. Of course, it wasn't supposed to lean when it was built; it was supposed to stand straight. It was intended as a bell tower for the church nearby. The church was begun in 1174 and finished in1350. The foundations of the tower were laid in sand, and this may explain why it leans. But it did not suddenly begin to lean—this began to happen when only three of its storeys had been built. So the plan was changed slightly and construction went right on! In the last hundred years, the tower has leaned another foot.