Read the following passage and mark the letter A, B, C, or D to indicate the correct answer to each of the questions.

At the University of Kansas art museum, scientists tested the effect of different colored walls on two groups of visitors to an exhibit of paintings. For the first group the room was painted white; for the second group, dark brown. Movement of each group was followed by electrical equipment under the carpet. The experiment showed that those who entered the dark brown room walked more quickly, covered more area, and spent less time in the room than people in the white one. Dark brown made people more active, but the activity ended sooner. Not only the choice of colors but also the general appearance of a room affects those inside. Another experiment presented people with photographs of faces whose energy was to be commented. Three groups of people were used; each was shown the same photos, but each group was in a different room. The first group was in an ordinary room and the second group was in a nice office. The third was in a tastefully designed living room with carpeting. Results showed that the people in the beautiful room tend to give higher marks to the facts than those in the ugly room did. Other studies suggest that the students do better on tests taken in comfortable room than in ordinary-looking or ugly rooms.

(https://www.philadelphia.edu.jo/academics/mjayyousi/uploads/sunday1-11B.pdf

The situation is serious. Something ____________ before it's too late. A. must do B. must be done C. must be doing D. must have done