A Nurse Notes That in Clients with Multiple Tattoos, Many

A nurse notes that in clients with multiple tattoos, many are missing quite a few of their teeth. The nurse measures the variables and establishes a negative correlation between number of tattoos and number of teeth. The nurse designs a study to measure causation. Why is this problematic?

A) Tattooing can be expensive.
B) Causing subjects to become multiply tattooed, so as to cause their teeth to fall out, is ethically questionable.
C) Random assignment to group would be impossible.
D) The nurse researcher cannot cause subjects' teeth to fall out, so as to later measure if their number of tattoos has increased; that would be ethically questionable.
E) The study would have to be designed as a large-sample, multisite design.