0
We know
[tex]\\ \rm\hookrightarrow Y=\dfrac{Stress}{Strain}[/tex]
[tex]\\ \rm\hookrightarrow Y=\dfrac{\sigma}{\Delta \ell/\ell}[/tex]
[tex]\\ \rm\hookrightarrow Y=\dfrac{0}{\Delta \ell/\ell}[/tex]
[tex]\\ \rm\hookrightarrow Y=0[/tex]