I will go with 'B'
I was also between 'B' & 'C' but I eliminated 'C' because the 'or' part is not mentioned in the passage
But how do u know that elevated levels of "nitrate" only by iteself
can fight bacteria, others factors may be involved also. I think the only thing we can conclude is that the cells cause the nitrate levels to rise, either by production of the same or inhibit the nitrate level to go down.