To feel relatively real in a virtual world, which one of the following has to be achieved according to the text?
   A．There have to be senses of smell and taste.
   B．Senses of smell and taste should be kept in step with senses of sight and hearing.
   C．Senses of sight and hearing have to be synchronized with the sense of touch.
   D．Senses of smell and taste have to keep pace with senses of touch and hearing.