Answer to Question 1
Gravitational lensing occurs when a nearby object (normally a galaxy) bends the light of a distant object (often a quasar), causing two or more images of the distant object to be seen. The lensing galaxy can have its mass calculated based on the amount by which it bends light. This lets us study dark matter by comparing the lensing galaxy's luminous mass with its gravitational mass.
Answer to Question 2
B