Which one of the following statements should you use to set the session username to IC$315?$

$$\_$SESSION$[\text{'}$username$\text{'}]="$IC$315"$;

session$\_$start$("$IC$315")$;

$SESSION$[\text{'}$username$\text{'}]="$IC$315"$;

$$\_$SESSION$\_$START$[\text{'}$username$\text{'}]="$IC$315"$;