Which of the following expressions corresponds to simple harmonic motion along a straight line, where $x$ is the displacement and $\mathrm{a},\mathrm{b},\mathrm{c}$ are positive constants?\\
(1) $a+bx-cx^{2}$\\
(2) $bx^{2}$\\
(3) $a-bx+cx^{2}$\\
(4) $-bx$