$2\mathrm{.}4$ Which of the following strings P is a Python program, according to the

formal definition given in the section $2\mathrm{.}5?($You may base your definition on any

reasonable reference computer C$.)$

$($a$)$ P $=$def f$($x$)$: return x$$

$($b$)$ P $=$def f$($x$)$: x $=\text{'}$am I a Python program?$\text{'}$

$($c$)$ P $=$x $=$ str$(7+2*5)$

$($d$)$ P $=$def f$($x$,$y$,$z$)$: return y$$

$($e$)$ P $=$def f$($x$)$: return y$$

$($f$)$ P $=$def f$($x$)$: ret x$$