Question 11 Answer and Explanation _ACT December 2009 Form 68A Science Section

Passage II

Figure 1 shows the life cycle of the malaria parasite. Gametocytes and sporozoites are 2 forms of the parasite.

For the parasite to survive, mosquitoes must bite humans infected with gametocytes. Therefore, 2 studies were conducted to determine whether mosquitoes are most attracted to humans infected with gametocytes.

Study 1

Three groups (Groups X, Y, and Z),each with 12 human subjects (Subjects 1-12), were identified (seeTable 1).

Three subjects—one from each group—rested in separate, but identical, tents of mosquito netting attached to the same central chamber. A mosquito trap was inserted between the chamber and each tent to capture any mosquitoes trying to enter the tent (see Figure 2).

In the first trial 100 uninfected mosquitoes were released into the chamber. Thirty minutes later the number of mosquitoes attracted to each subject was determined. This procedure was repeated in 11 more trials until all subjects had been tested. Then, for each group, the average number of mosquitoes attracted to a subject was calculated (see Figure 3) . All infected subjects were then treated to cure their sporozoite or gametocyte infections.

Study 2

The procedure from Study 1 was repeated with Group X subjects and the cured subjects from Groups Y and Z. Figure 4 shows the proportion of responsive mosquitoes (those caught in a trap) that were attracted to the Group Z subjects before treatment and after being cured of their gametocyte infections.

Figures adapted from Renaud Lacroix et al., "Malaria Infection Increases Attractiveness of Humans to Mosquitoes." @2005 by Renaud Lacroix et al.

11. Which of the following procedures was most likely employed to ensure that the mosquitoes were attracted to a specific subject rather than to a specific tent?

A. Releasing the same number of mosquitoes into each tent in each trial

B. Releasing a different number of mosquitoes into each tent in each trial

C. Assigning the subjects to tents in a random manner in each trial

D. Assigning the gametocyte subject to the same tent in each trial

Answer and Explanation

Your Answer is

Correct Answer is C

Explanation

Examine the common sense of the experimental setup. In general experiments, the method of randomly arranging subjects will be used to avoid the influence of other interference factors on the experimental results Other chemical signals will attract mosquitoes, and this chemical signal may remain in the tent, which will cause mosquitoes to be attracted by this chemical signal after the people behind enter the tent again, not necessarily attracted by a specific subject )