Tsumego (Capture) #5309437 (2-move)
Black to play — capture the marked white stones
Capture these stones
Why this move works
The answer is C2 then A2. The three white stones (A1, B1, B2) have liberties at A2 and C2.
Play C2 first. If you start with A2, White runs out to C2. Stop the extension at C2, then fill A2 to capture.