how long have you been dancing today?

how long have you been dancing today? How long have you been dancing today?为什么要加been!而T How long have you been in class today?怎么翻译? ---How long have you been in the class today?-----_________. how long did you skate today?和how long have you been skating 英语翻译How long did you skate today?How long have you been skat "How long have you been how,have,in,today,class,long,you,been,for,hours,twelve,over如 How long have you been skating? how long have you been playing basketball ? How long have you been swimming? How long have you been ____?