Certain messenger molecules fight damage to the lungs from noxious air by telling the muscle cells encircling the lungs' airways to contract. This partially seals off the lungs. An asthma attack occurs when the messenger molecules are activated unnecessarily, in response to harmless things like pollen or household dust. Which of the following, if true, points to the most serious flaw of a plan to develop a medication that would prevent asthma attacks by blocking receipt of any messages sent by the messenger molecules referred to above? ['Researchers do not yet know how the body produces the messenger molecules that trigger asthma attacks.', 'Such a medication would be a preventative only and would be unable to alleviate an asthma attack once it had started.', 'Such a medication would be unable to distinguish between messages triggered by pollen and household dust and messages triggered by noxious air.', 'Such a medication would not become available for several years, because of long lead times in both development and manufacture.']