Could the magnetic fields from power lines cause leukemia in children? Investigators spent five years comparing 638 children who

Could the magnetic fields from power lines cause
leukemia in children? Investigators spent five
years comparing 638 children who had leukemia
and 620 who did not. They went into the homes
and measured the magnetic fields in the
children’s bedrooms. This is an example of:
A. An Observational Study
B. A Controlled Experiment
C. A Survey