What is the definition of imperceptible?

The excerpt above is from page 21 of harry potter and the sorcerers stone by j.k rowling. which passage represents a

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The story line remains the same. the kids live in what was then one of the remaining beachfront slums, down the coast from the expensive malibu area. the beach was ruled by surfers, but in the afternoon, when the waves died down, some of the surfers, or their younger brothers, fooled around on skateboards. one day, skip engblom, the shop owner, comes up with a key breakthrough, polyurethane wheels: "they grip." with the additional traction, the z-boys try skating the sides of the big, open drainage canal that runs through the area. then comes a brainstorm: because of a drought, the area's swimming pools were drained. the kids started "borrowing" pools when the owners weren't home, to skate the curved sides.” where did “the lords of dogtown” originally practice their tricks? a. the beach c. an open drainage canal b. empty pools d. none of these