Suppose you are interested in how subjects perform under different temperature conditions. Subjects work in one of three rooms. The rooms differ only with respect to temperature. You instruct all subjects to solve the same jigsaw puzzle. You then measure how long it takes each subject to finish. In the research described above, differences in temperature conditions serves as which of the following:
observation
internal attribute
independent variable
dependent variable

Nebulae are enormous clouds in outer space. they are made mostly of hydrogen gas, helium gas, and dust. some nebulae glow brightly, while others do not. the stars that people see are huge, bright balls of glowing gas. they are made mostly of hydrogen and helium. which statement correctly describes other ways in which nebulae and stars are different? a. stars can form inside a nebula but a nebula can never be produced by any star. b. a star always has a higher density than a nebula. c. stars can never form inside a nebula but a nebula can be produced by any star. d. a nebula always has a higher density than a star.