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To solve the exercise, we need to fill in the blanks with the most appropriate words from the provided options based on the context of the passage.

1. 31: "this" or "that" is needed here. In this context, "this" is preferred since the previous sentence introduces the subject matter being discussed. Thus, "this" is the answer.

2. 32: We need a word that indicates awareness or consciousness. "aware" fits best since it refers to the teenagers realizing or being conscious of the number of illnesses.

3. 33: The word should relate to the frequency of minor illnesses. "fewer" is the correct choice, as it indicates a smaller number of occurrences.

4. 34: Here, we look for a word that deals with the amount of sleep. "longer" suits this case best because it suggests a greater duration of time sleeping.

5. 35: We need a word that fits the context of the problem mentioned in the study. "producing" works well as it refers to the effect or outcome of the sleeping habits on their ability to wake up, hence is the best choice.

Combining all these analyses, the final answers are:
31: A. this
32: B. aware
33: D. fewer
34: C. longer
35: B. producing