L A Nielson \"set metel\" is an electronic device placed in people's homes that show what their television watching habits are. You want to study the viewing habits of residents of your neighborhood. Which study is observational? A " The experimental group consists of those who asked for the device, and the control group consists of those who did not. A The neighborhood is divided into blocks and all houses in randomly selected blocks are given the device. The experimental group is those who receive the device. A Households are sorted by their account numbers and every 101h household is given the device. The experimental group is those who receive the device. A U You randomly assign some household to receive the device. The experimental group is those who receive the device