在气焊、医疗、登山等生产、生活中都要用到的一种气体是（ ）
A．氧气
B．一氧化碳
C．氢气
D．氮气
答案：A
Our town has been added to these years. It is no longer ________ it was 15 years ago, ________ it was poor and untidy.
A．what; which
B．that; which
C．which; that
D．what; when
板式输送机的拉紧装置通常采用（）。
A.车式拉紧装置
B.垂直式拉紧装置
C.螺旋拉紧装置
