Two things to keep in mind when you’re deciding whether to use formal or informal english are?

a. the opinion and the occasion.
b. the reader and the location.
c. the occasion and the audience.
d. the author and the audience.

A. The opinion and the occasion.

D. The author and the audience.

Explanation:

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1.the hosts on two opposing mountains stood,  

2.thick as the foliage of the waving wood;  

3.between them an extensive valley lay,  

4.o'er which the gleaming armour pour'd the day,  

5.when from the camp of the philistine foes,

6. dreadful to view, a mighty warrior rose; in the dire deeds of bleeding battle skill'd,