【单选题】With the efforts to analyze the atmosphere in the 17th and 18th century, _____ such as hydrogen were discovered which in turn led to the invention of hydrogen balloons .
【单选题】With the efforts to analyze the atmosphere in the 17th and 18th century, ______ such as hydrogen were discovered which in turn led to the invention of hydrogen balloons.
【单选题】With the efforts to analyze the atmosphere in the 17th and 18th century, _____ such as hydrogen were discovered which in turn led to the invention of hydrogen balloons .