有四根长都为2的直铁条，若再选两根长都为 $a$的直铁条，使这六根铁条端点处相连能够焊接成一个三棱锥形的铁架，则 $a$的取值范围是(   ).

A．	$(0,\sqrt{6}+\sqrt{2})$	B．	$(1,2\sqrt{2})$
C．	$(\sqrt{6}-\sqrt{2},\sqrt{6}+\sqrt{2})$	D．	$(0,2\sqrt{2})$