$2.($a$)$ List conditions for any function to be a valid ACF. Note that all the conditions are necessary but not sufficient individually. That means to prove that a function is an ACF, all conditions should be satisfied. Even if one of the conditions is not satisfied, it is not a valid ACF. $($b$)$ Now, for each of these functions, determine whether they are valid ACFs or not, with appropriate justification.

cos$()$ e$228$

$($i$)$ ; $($ii$)$ and $($iii$)2+4.$