Please use Big$-$O notation to represent the time complexity of the following code:

for $($i$=0$; i$<$ strlen$($str$)$; i$++)$

$\{$

c $=$ str$[$i$]$;

cout $<<$ c;

$\}$

A$.$

O$($logn$)$

B$.$

O$($n$2)$

C$.$

O$($n$3)$

D$.$

O$($n$)$