A company was testing a new drug it thought would help decrease the risk of transmission of viruses from mother to fetus. In an experiment to test the compound, an investigator gave 400 pregnant female rats a small dose of the experimental drug and inoculated each with a type of virus known to cause disease in rats. At the same time, 400 other pregnant rats were given only the virus. Of the rat pups born to the females that received both the virus and the drug, 197 showed no symptoms of the disease; 205 rat pups born to the virus-only females showed symptoms. From these data, we can best conclude __________.

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