If kill the Thrasher before talking about him with D'Flinger, you can tell him that you have killed the crab 3 times:
- I've found Thrasher. It's a dead hermit crab -> (any option)
- What's under that manhole in the middle of the room? -> Thrasher? You meant that giant hermit crab? -> (any option)
After the option above third option appears:
- Tell me about Thrasher -> I know what this is. It is a giant hermit crab -> (any option)
Certain options can appear without talking to him about Thrasher first.