A team of scientists uses the Hubble Space Telescope to search for evidence of planets orbiting distant stars. Which kind of investigation are they carrying out?

Which question would be appropriate for an observational study?

A scientist wants to determine how the speed of flowing water affects the size of sediment particles it can carry. The scientist places a mix of sediment particles in the bottom of a trough and runs water through the trough. The scientist varies the speed of the water and measures the sizes of the particles that the water carries away. Which type of investigation is the scientist using?

Which question would be appropriate for an observational study?

Numerous experiments have confirmed that if the distance between a light source and an observer is doubled, the intensity of the light as viewed by the observer decreases by a factor of four. Which type of scientific statement is this?