sin A + cos A = k: find sin^4 A + cos^4 A

1. A.
   $\frac{5}{13}$
2. B.
   $\frac{79}{128}$
3. C.
   $\frac{79}{126}$
4. D.
   $\frac{7}{13}$

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1. A.
   $1$
2. B.
   $\frac{1}{2}$
3. C.
   $\frac{3}{4}$
4. D.
   $\frac{5}{8}$

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