c
100 percent sure
ax+by+ c can be a normal to xy=1 if
a] a>0 , b<0
b]a<0 , b>0
c] a and b are of opposite signs
d] a=0 , b=0
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ax+by+ c can be a normal to xy=1 if
a] a>0 , b<0
b]a<0 , b>0
c] a and b are of opposite signs
d] a=0 , b=0