下列哪些是活疫苗的特点（）。

A.接种剂量较小

B.一般只需接种一次

C.免疫效果较好

D.疫苗较易保存

E.模拟自然感染途径

MIDI enables people to use （66） computers and electronic musical instruments. There are actually three components to MIDI, the communications " （67） ", the Hardware interface and a distribution （68） called "Standard MIDI Files". In the context of the WWW,the most interesting component is the （69） Format. In principle, MIDI files contain sequences of MIDI Protocol messages. However, when MIDI Protocol （70） are stored in MIDI files, the events are also time-stamped for playback in the proper sequence. Music delivered by MIDI files is the most common use of MIDI today.

A．format

B．text

C．wave

D．center