$(16$ points$)$ Creep experiment is performed with an idealized soft material at a constant stress ${}_0=25$MPa. The strain values of $0\mathrm{.}108$ and $0\mathrm{.}156$ were measured after $30$ and $60$ minutes, respectively. Assuming theat the creep experiment follows Kelvin$-$Voight model,

$($a$)$ Calculate the elastic modulus $($E$)$ of the material. $(12$ points$)$

$($b$)$ Plot the strain response curve for this material and determine the maximum limiting strain that can be achieved with this material under the stress of $25$ MPa $.(4$ points$)$