One of the advantages of Bacillus thuringiensis (B. t. ) toxins over chemical insecticides results from their specificity for pest insects. The toxins have no known detrimental effects on mammals or birds. In addition, the limited range of activity of the toxins toward insects means that often a particular toxin will kill pest species but not affect insects that prey upon the species. This advantage makes B. t. toxins preferable to chemical insecticides for use as components of insect pest management programs. Which one of the following statements, if true, most weakens the argument? ['Chemical insecticides cause harm to a greater number of insect species than do B. t. toxins.', 'No particular B. t. toxin is effective against all insects.', 'B. t. toxins do not harm weeds that do damage to farm crops.', 'Insects build up resistance more readily to B. t. toxins than to chemical insecticides.']