Read the following passage carefully before you choose your answer. This passage is excerpted from a medical article titled, "Electronic cigarettes: Not all good news?"

(1) There is a long history of deceptive marketing tactics used by the tobacco industry regarding the 'safety' of cigarettes. (2) Thus, it is interesting to speculate whether the same will hold true for the nascent E-Cig industry.

(3) Certainly, users want to believe that E-Cig products are safe, but, unfortunately, no definitive data currently exist to prove or disprove this hypothesis. (4) Studying E-Cig exposure is very much like trying to hit a moving target, but one where researchers are not completely sure what the target looks like. (5) However, some facts have been established: (a) current E-Cig devices deliver nicotine at comparable levels to cigarettes, and certainly at levels high enough to evoke physiological responses in humans and rodents; (b) nicotine is highly addictive and, along with its metabolites, can cause cancer and affect neuronal development in adolescents irrespective of its source; (c) e-liquids have been shown to contain potentially toxic aldehydes and ROS, and; (d) some type of a biological response (e. g., change in cytokine levels) has been observed in the vast majority of murine in vivo and in vitro studies following E-Cig vapor/e-liquid exposure.

(6) Although it seems certain that E-Cig aerosols contain toxicants, it is fair to say that they likely contain less types of toxicants than cigarette smoke (i. e., E-Cig aerosols likely have hundreds of chemicals in them while tobacco smoke has thousands of chemicals). (7) The remaining question is then one of dose ranging.

(8) Given the paucity of information that is available regarding the effects, not only of E-Cigs, but also of many of the chemical constituents of e-liquids on the lungs, we propose that all commercially available E-Cig products be regulated in a similar fashion as any inhaled therapeutic agent, that is, thorough inhalation toxicology and safety-based clinical trials. (9) Although this would be an undeniably expensive undertaking, the estimated value of the E-Cig market is in the billion-dollar range, indicating that tobacco and E-Cig companies could likely foot this bill.

Which of the following phrases, if placed at the end of sentence 4, would help the reader to further understand why E-Cigs are "a moving target"?

since E-Cig devices appeal most heavily to younger users who are less likely to be truthful in reporting their use.
since E-Cig devices are increasing in popularity and the number of them sold each year increases.
since E-Cig devices are only available in a few states and none of them have reliable data to reference.
since E-Cig devices have a consistent and dedicated number of users which remains rather constant.
since E-Cig devices, the way that they are used, and the types/flavors of e-liquids available are constantly changing.