There are three metal balls, arranged by mass. A ball is the heaviest, followed by B ball, C ball is the lightest, and there is a D ball. Please weigh it twice with the balance to determine the weight ranking of D ball.
This is very simple, because knowing three balls, you only need to compare D with the intermediate quality B ball:
1. If D is heavier than B, compare it with A.. If D is heavier than a ball, its ranking is 1, that is, D>A & gtB& gt；; c； If it is lighter than the ball, it ranks second, that is, A>D & gtB& gt；; C
2. If D is lighter than B ball, compare it with C ball. If the D ball is heavier than the C ball, it ranks third, that is, A>B >; D & gtc； If it is lighter than the C ball, it ranks fourth, that is, A & gtB& gt；; C & gtD