$1-1-6-12$: Given the following code segment, what will the value of $s1$ be after this executes?

String $s1=$ HiThere;

String $s2=s1$;

String $s3=s2$;

String $s4=s1$;

$s2=s2.$tolowerCase$()$;

$s3=s3.$toUpperCase$()$;

$s4=$ null;

A$.$ Hi There

B$.$ hi there

C$.$ HI THERE

D$.$ null

E$.$ hl tHERE