--- The little girl dances so well. Where was she trained?

-- She learns all by herself. She ______ goes to any training class.

A．usually

B．often

C．even

D．never

已知有¹H₂¹⁶O、D₂S、³H₂¹⁸O、S¹⁷O₂、²H₂S、S¹⁶O₂，其中有______种元素，______种核素，_____种分子。