Our school was going to send a teacher to Xinjiang . I was the first person _______ to go.

A．volunteer

B．to volunteer

C．volunteered

D．volunteering

以目视法能观测到酸碱指示剂颜色变化时，其酸式体与碱式体的浓度比应是

A．2∶1或1∶2
B．5∶1或1∶5
C．10∶1或1∶10
D．15∶1或1∶15
E．20∶1或1∶20