Explanation:

A-S; B-R; C-P; D-Q

A cereal is any grass cultivated for the edible components of its grain, composed of the endosperm, germ, and bran. The term may also refer to the resulting grain itself. 

A legume is a plant in the family Fabaceae, or the fruit or seed of such a plant. When used as a dry grain, the seed is also called a pulse. Legumes are grown agriculturally, primarily for human consumption, for livestock forage and silage, and as soil-enhancing green manure. 

Oilseeds are rich in protein, and in addition they contain a high level of fat. Hence, they are not only good sources of protein, but, also concentrated source of energy. The proteins in oilseeds can be fed either as part of the oil-intact seed, or as a meal from which the oil has been removed.