A、Space particles. B、Precious stones. C、Plastic. D、Metal. C细节题。在对话的第三句中说到“tiny plastic beads”，所以应选C)。

M: Did you know that astronauts have made commercial products in space?
W: No, I wasn’t aware of that. What kind of products?
M: Tiny plastic beads with a little hole through it.
W: Tiny balls? Do you mean to tell me that astronauts have nothing better to do than make jewelry beads in space? It seems to me they could make more useful things out there.
M: Oh, but these little balls aren’t for jewelry. They can be used for scientific purposes to conduct cancer research .
W: That sounds better, but why make such objects in space rather than on earth?
M: Because earth gravitational pull affects the beads. They produced on earth are distorted, not exactly tumid. The ones made in space are precisely round.
W: Sounds reasonable.