Viruses cause disease by destroying and by disrupting the body's natural homeostasis.
A. cells
B. influenza
C. tissues
D. organs

The answer is A, cells, as viruses infiltrate cells, make copies of themselves in them, and then lyse the cell and spread.

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.