Standing on the shore of a lake, I couldn't help admiring the hundreds of small rocks that___11____(surround)my shoes. All created from hard surfaces, the rocks softened over time. And I was wondering __12__we could learn from a pile of rocks. Once was a sharp rock that was covered with pointy edges, and now I have found my attitude have softened and my desire to understand others ___13___(expand)with time.

B

Oil dominates every aspect__14__our life. It fuels our cars, and it is used in production of our plastic goods and the electricity for our homes and factories. Moreover, it can even ___15___(find) in the fertilizer for the soil in which our food grows. Oil makes nations ___16___ (extreme) wealthy, while shortages of oil can bring economies to their knees. In short, this black and sticky substance has been running__17___(we)world.

C

When we go outdoors in winter, cold wind gets through our clothes, __18____(hit)our body parts fiercely. Then we may feel cold. How do we feel cold? Actually human skin is filled with temperature-sensing nerve receptors named TRPs, which receive stimuli （刺激）from the environment. ___19___（sense）the cold，the brain uses one of the receptors called TRPM8.When the TRPM8 is hit by stimuli, it will send an electric signal to the brain,__20___understands the signal as "cold".