为解决登山人员的呼吸困难，需提供的物质是（ ）
A．氮气
B．二氧化碳
C．氧气
D．水
答案：C
已知x=3是方程x+2m=5的解，则m=______．
. --- Did you often e-mail your family during the first year at university?
--- No. I was so busy meeting new people that I didn’t write to my family as I _____.
A．ought to
B．should do
C．should have written
D．should have
