Which of the following is not associated with purines? Adenine
DNA
single ring structure
Guanine

Single ring structure is not associated. Double ring structure is a Purine and single ring structure is a pyrimide so they are two different things. Purine is associated with DNA because the bases Adenine and guanine in DNA are purines.

sheep is a herbivore animal whereas grasshopper belongs to the class chordates but both of them feed on greens.  

if grasshopper and sheep feeding on a pasture land, i do not think both of them will have any impact on each other. however, the number of the species can make difference. since sheep feed on greens, there is a possibility there left no pasture land for the grasshoppers or they may get extinct.