This kind of problem should belong to the difficulty level of junior high school mathematics. MBA exams are generally people who have worked for many years, so the difficulty is generally not great. Math exams are all elementary math.

On this issue, assuming that the volume of the container is V, the concentration becomes:

(09.V-0.9)/V

After the second filling and filling, the concentration becomes:

[0.9V-0.9-(0.9V-0.9)/V]/V = 0.9(V- 1)？ /V？ V=3 can be obtained by =0.4, so the answer is B. Regarding the real questions related to MBA, I suggest you read the book "Analysis of Mathematics Real Questions over the Years" written by Teacher Shi Peng, and the answer is still very detailed.