题目

About ______ of the land ______ covered with trees and grass．       A．three fifths； is                 B．three fifths； are       C．three fifth； are                 D. three fifth ;    is 答案：A NH4HCO3在试管中加热，使放出的气体依次通过盛有足量过氧化钠的干燥管，足量浓H2SO4的洗气瓶，最后得到的气体是 A． NH3 B． O2 C． H2O D． CO2