As a defense against air-pollution damage, many plants and animals ________ a substance to absorb harmful chemicals.
句意：为了防止空气污染，许多动植物都释放一种可以吸收有害化学物质的物质。release释放；发射。relieve解除；减轻。 dismiss解散；开除。discard抛弃。因此，本题的正确答案为C。