Given the following code $($with line numbers provided for clarity$)$:

$1$: def readAge$($filename$)$:

$2$: infile $=$ open$($filename$)$

$3$: strAge $=$ infile.readline$()$

$4$: age $=$ year $-$ int$($strAge$)$

$5$: return 'born in $\{\}\text{'}.$format$($age$)$

Which lines could possibly throw an exception?