There is very simple way of measuring the height of a water-tower, which we cannot measure by climbing. Suppose, for example, that we wish to find out the height of the water-tower, AB, in a factory. We first of all go to where the water-tower is standing and measure a distance of, say, 25 feet from it. Then we take a stick, and stand it in the ground at the spot we have just marked. Let us suppose the stick we are using is 4 feet in height. We now walk farther away from the water -tower in the same straight line as when we measured off the distance of 25 feet. We go from the water-tower until we come to point E, where with our head on the ground, we see the top of the stick and the top of the water-tower in the same height line---that is, the top of the stick just covers the highest part of the water-tower. Every schoolboy can work out the height of the water-tower now. Suppose that the line CE is five feet. We know that the stick is 4 feet high and the distance BE is 30 feet. Thus, 5 is to 4 as 30 is to AB. It is clear from this that the problem can be settled by   _  .
a child the boy who has little schooling everyone a girl at middle school