A recent study by an insurance company showed that those who work in a comfortable work environment are 25% more productive than those who work in an uncomfortable work environment. This shows that an increasingly improved work environment can increase worker productivity. Which of the following hypotheses is the most serious challenge to the above causality? ['On average, less productive employees spend less time in the workplace than more productive employees.', 'A comfortable environment is more motivating than hard work.', 'A comfortable work environment is usually a reward for highly productive employees.', 'In a crowded, uncomfortable environment, coworker pressure prevents employees from working.']