A recent study found that snoring, though not common in either group, is more common among smokers than among nonsmokers. On the basis of this evidence, the author hypothesized that smoking by itself can induce snoring. Which one of the following, if true, casts the most doubt on the author's hypothesis? ['Most snorers do not smoke.', 'Stress induces both snoring and smoking in certain individuals.', 'Most smokers do not snore.', 'Both smoking and snoring cause throat problems.']