6. Arrange all natural numbers that are coprime with 70 in ascending order, forming a sequence $\left\{a_{n}\right\}, n \geqslant 1$. Then $a_{200}$ equals ( ).
(A) 568
(B) 583
(C) 638
(D) 653
6. B

Translate the text above into English, please retain the original text's line breaks and format, and output the translation result directly.