Given this function definition:

def plus$\_$two $(x)$ :

return $x+2$

which of the following code fragments contain valid calls to the function? Select all correct choices.

$x=7$

$y=$ plus$\_$two$()$

plus$\_$two $(6,y)$

$y=$ plus$\_$two$($plus$\_$two $(6))$

plus$\_$two $(5)$

$x=3$

$x=$ plus$\_$two $(x)$

$a=5$

$y=$ plus$\_$two $(a)$