1.Aspirin is a simple but highly ______treatment. 2. Light clothes are _______ in keeping cool in warm weather. 3. Mary turned out to be an secretary. 4. As we get older, our bodies become less _______at burning up calories.
【单选题】The first two stages in the development of civilized man were probably the invention of weapons and the discovery of fire, although nobody knows exactly when he acquired the use of the ____.