Which of these sentences from this passage shows a shift in the voice?
a. Hoarders are people who are unable to throw away material things because of an attachment.
b. In addition, it takes them a long time to decide about throwing away something.
c. Hoarding has been seen as a serious problem in America and researchers have studied it in detail.
d. In between the periods of decision-making, they collect more stuff that they have to think about.