\input epsf
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\magnification=\magstep1

\noindent {\bf Problem statement} Below are the graphs of three
functions $\,y = f(x).$ In just one of the graphs, it is true for all
$x$ that $\displaystyle{ { {d^3\!y} \over {dx^3} } > 0 }.$ Which is
the graph? Explain why the other two graphs could
not possibly satisfy the condition $\displaystyle{ { {d^3\!y} \over
{dx^3} } > 0 }$ for all $x$.

\medskip

\centerline{(A)\hskip 1.6in (B)\hskip 1.6in (C)}

\centerline{\epsfxsize=5in\epsfbox{nan18.eps}}
     
%contributed by Michael O'Nan

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