What Is the Microlensing Technique

What is the microlensing technique?

A) A small telescope is placed in orbit around Earth to observe extrasolar planets.
B) A planet that passes between Earth and some distant star focuses the starlight and causes an increase in its apparent brightness.
C) Refracting telescopes for use in the search for extrasolar planets are constructed with very small lenses to avoid the problems of large refraction telescopes.
D) A planet passing between Earth and its star focuses the light from the star and makes it appear to brighten.