Which line of code represents the correct syntax to establish a reference to a database handle?

Which line of code represents the correct syntax to establish a reference to a database handle?

A.
$dbh = DBI::connect(“dbi:mysql:myPhoneBook”);

B.
$dbh = DBD:->connect(“dbi::mysql::myPhoneBook”);

C.
$dbh = DBD::connect(“mysql:dbi:myPhoneBook”);

D.
$dbh = DBI->connect(“dbi:mysql:myPhoneBook”);