A Psychology Professor Is Conducting an Experiment in Which Subjects

A psychology professor is conducting an experiment in which subjects perform memory tasks. There are four groups, each participating in different buildings on campus. During the time of the experiment one group was in a building near a dumpster that was being emptied by a big, noisy trash truck. The other groups weren't close enough to hear the noise. Which type of error does this introduce into the research?

A) systematic error
B) sampling error
C) testing error
D) interference error