A Researcher Is Interested in Environmental Effects on Workers' Productivity

A researcher is interested in environmental effects on workers' productivity. Each of 20 call-centre workers is tested twice, one day in a noisy open-plan office and the next day in a quiet individual office. A record is made of how many calls they manage to process in each day. What is the major problem with this study?

A) The lack of counterbalancing means that order effects may affect the data.
B) Number of calls processed is a poor measure of performance.
C) Twenty call-centre workers is too few for a study like this.
D) The study hasn't identified exactly what sorts of environmental factors might have produced the results.