Some contact lens wearers report problems with dryness in their eyes. A study was conducted to evaluate the effectiveness of a new eye-drop solution to relieve dryness for contact lens wearers. Twenty-five volunteers who wore contact lenses agreed to use the new solution for one month. At the end of the month, 36 percent of the volunteers reported that the new solution was effective in relieving dryness. The company that produced the new eye-drop solution concluded that using the new solution is more effective in relieving dryness than using no solution. Which of the following best explains why the study does not support such a conclusion.

(A) The sample size was too small.
(B) The study had no control group.
(C) The participants were volunteers.
(D) The participants self-reported the frequency with which they used the new solution.
(E) The participants self-reported the effectiveness of the new solution.