Answer:
a.
Explanation:
A catenary looks a lot like a parabola. Only graph "a" has that appearance.
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A graphing calculator can help you choose, or you can recognize the nature of the terms of the sum.
e^x looks like graph D; e^-x looks like graph B. Their sum will always be positive, so cannot create graph C. At x=0, the average of the two graphs B and D will be 1, corresponding to the minimum of graph A.