The practice of hanging clothes across the street is a common _____ in many parts of the city. [ ]

A. look

B. sign

C. sight

D. appearance

我国儿童服用的脊灰疫苗为下列哪一种？（）

A．灭活疫苗

B．减毒活疫苗

C．亚单位疫苗

D．DNA疫苗

E．合成疫苗