The next two questions refer to the following passage:
A pharmaceutical company tested a new painkiller on 1,000 lab rats that were fed large doses of the painkiller for a two-month period. By the end of the experiment, 39 of tile rats had died. The company concluded that the painkiller was sufficiently safe to test on humans.
Which of the following, if true, would cast the most doubt on the pharmaceutical company’s conclusion ？（）
A. Fifteen of the 39 dead rats were later found to have died of a rare form of liver cancer that is generally seen in only 1 out of 600,000 lab rats.
B. The director of the experiment, although a respected scientist, is not a medical doctor, and therefore will not be able to continue as director of the experiment once it switches from animal to human trials.
C. Although the painkiller in question has been shown to block certain forms of pain, its duration of efficacy is less than that of other painkillers currently on the market.
D. Human trials of another drug produced by this pharmaceutical company were called off after 30 out of 1,000 rats died of heart failure during the animal testing phase of the experiment.
E. The pharmaceutical company is eager to introduce new drugs to the market because its best-selling product, a drug that reduces blood pressure, will soon lose its patent protection.