Relief work and selfless ______ aid continues, which helps deliver needed equipment to the disaster area.

A．conventional

B．unusual

C．outstanding

D．voluntary

锅炉负荷大于25%额定负荷时，至少每（）小时对回转式空气预热器吹灰一次，锅炉负荷低于25%额定负荷时，应坚持对回转式空气预热器进行（）吹灰。