<P> The following horizons are listed by their position from top to bottom within the soil profile . Not all of these layers are present in every location--for instance, P horizons only form in areas which have been waterlogged for long periods of time . Soils with a history of human interference, for instance through major earthworks or regular deep ploughing, may lack distinct horizons almost completely . When examining soils in the field, attention must be paid to the local geomorphology and the historical uses to which the land has been put in order to ensure that the appropriate names are applied to the observed horizons . The horizon not listed is the O horizon which is grass and animal / plant life . Soil has three main horizons (A, B, and C), which will be explained below along with other layers . </P> <P> Soil generally consists of visually and texturally distinct layers, which can be summarised as follows from top to bottom: </P> <Table> <Tr> <Td> </Td> <Td> O) Organic matter: Litter layer of plant residues in relatively undecomposed form . <P> A) Surface soil: Layer of mineral soil with most organic matter accumulation and soil life . This layer eluviates (is depleted of) iron, clay, aluminium, organic compounds, and other soluble constituents . When eluviation is pronounced, a lighter coloured "E" subsurface soil horizon is apparent at the base of the "A" horizon . A-horizons may also be the result of a combination of soil bioturbation and surface processes that winnow fine particles from biologically mounded topsoil . In this case, the A-horizon is regarded as a "biomantle". </P> <P> B) Subsoil: This layer accumulates iron, clay, aluminium and organic compounds, a process referred to as illuviation . </P> <P> C) Parent rock: Layer of large unbroken rocks . This layer may accumulate the more soluble compounds . </P> R) bedrock: R horizons denote the layer of partially weathered bedrock at the base of the soil profile . Unlike the above layers, R horizons largely comprise continuous masses (as opposed to boulders) of hard rock that cannot be excavated by hand . Soils formed in situ will exhibit strong similarities to this bedrock layer . </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> O) Organic matter: Litter layer of plant residues in relatively undecomposed form . <P> A) Surface soil: Layer of mineral soil with most organic matter accumulation and soil life . This layer eluviates (is depleted of) iron, clay, aluminium, organic compounds, and other soluble constituents . When eluviation is pronounced, a lighter coloured "E" subsurface soil horizon is apparent at the base of the "A" horizon . A-horizons may also be the result of a combination of soil bioturbation and surface processes that winnow fine particles from biologically mounded topsoil . In this case, the A-horizon is regarded as a "biomantle". </P> <P> B) Subsoil: This layer accumulates iron, clay, aluminium and organic compounds, a process referred to as illuviation . </P> <P> C) Parent rock: Layer of large unbroken rocks . This layer may accumulate the more soluble compounds . </P> R) bedrock: R horizons denote the layer of partially weathered bedrock at the base of the soil profile . Unlike the above layers, R horizons largely comprise continuous masses (as opposed to boulders) of hard rock that cannot be excavated by hand . Soils formed in situ will exhibit strong similarities to this bedrock layer . </Td> </Tr>

Which of the following is the designation for bedrock a. o b. a c. r d. c