# The curve $y=e^{-x}$ is

$\begin{array}{1 1}(1)concave\;upward\;for \;x>0&(2)concave\;downward\;for\;x>0\\(3)every \;where\;concave\;upward &(4)every\;where\;concave\;downward\end{array}$

$y=-e^{-x}$
$\large\frac{dy}{dx}$$=-e^{-x} x-1=e^{-x} \large\frac{d^2y}{dx^2}$$=-e^{-x} <0$ for all x
The given curve is concave downward every where
Hence 4 is the correct answer.