(a) In a food chain the energy moves progressively through the various tropic levels and is no longer available to the organisms of the previous trophic level/ energy captured by the autotrophs does not revert back to the solar input.
(b) Pesticides used for crop protection when washed away/ down into the soil/ water bodies absorbed by plants/ producers.
(c) On consumption they enter our food chain and being non-biodegradable these chemicals get accumulated progressively and enter our body.