Check each option by adding the parts.<br><br>
&bull; A: 6000 + 700 + 20 + 1 = 6721. Equal!<br>
&bull; B: 6000 + 600 + 120 + 1 = 6721. Equal!<br>
&bull; C: 6700 + 21 = 6721. Equal!<br>
&bull; All three options equal 6721.<br><br>
Final Answer: D: All of the above