Each of two boxes contain 20 red balls and 30 black balls. We select a ball from the first box randomly and place it into the second one. Then, we select a ball from the second box and place it into the first one.

A: both balls selected were red;

B: the ball selected from the second box was black;

C: at the end of the above experiment, each box contains the same number of balls from each color as in the beginning.