Read the passage and then answer the question. Most known planets in the Milky Way orbit stars, but there...

Read the passage and then answer the question.
Most known planets in the Milky Way orbit stars, but there are certain "rogue planets" that are not bound to a star and simply orbit the gravitational center of the galaxy. Such planets can be difficult to find. Astronomers' usual methods for identifying a planet outside our own solar system rely on observing the effect of the planet on the star it orbits; since they lack a host star, many rogue planets are difficult to detect. However, astronomers can sometimes locate an otherwise undetectable planet when its gravitational field briefly magnifies incoming light from more distant stars. A rogue planet may also emit enough radiation to be detected through infrared imaging. Which of these two approaches is most appropriate depends on the characteristics of the object being observed: microlensing works best to expose more massive objects, while infrared imaging is most effective for hot ones.
What is the meaning of microlensing as used in the passage?
A. a method of observing objects based on how their gravity influences the positions of the stars they orbit
B. a method of observing objects based on how their gravity affects light from a background source
C. a method of observing objects based on the amount of infrared they radiate
D. a method of observing objects based on their temperature