In the scale adsorption of a magnet is equivalent to change the gravity of the scale, gravity becomes larger, the force arm becomes smaller, the number of readings from the scale is smaller, will make the measured value is less than the true mass of the object. So A, D is wrong, B is correct;
When using B to lift the button, change the position of the fulcrum, the equivalent of the object's force arm becomes larger, the scale's maximum weighing value than the maximum weighing value when using A to lift the button is small. So C is wrong.
The answer is B.