The explanation in the plot is this:

1. The probability that she is in three classrooms A/B/C is 1/3.

2. So, the probability that she is in classroom A is 1/3.

3. The possible line of classroom B or classroom C is 1/3+ 1/3 = 2/3.

4. Later, because classroom B was opened and there was no one in it, the possibility of being in classroom B was zero.

5. As mentioned in Article 3, the possibility of being in Class B or Class C is 2/3, and the possibility of being in Class B is 0/4.

So the possible line of classroom C is 2/3.

In fact, because of the inference of article 5, I think it is unreliable.

When classroom B is empty, the probability that classroom A or C is occupied should be 1/2.