A student hypothesized that robins prefer large birdhouses to small ones. He build four birdhouses of different sizes to test his hypothesis. What was the independent variable in the students study?

Structures in cells that carry out specific functions.

The circles, or orbits, for electrons are called energy levels. each level can hold only a certain number of electrons. add electrons to each level until you can't add any more. how many electrons can each level hold?

In which situations is the principle of cross-cutting relationship useful in determining relative age?