A、When they started living in trees. B、When they started making a tree house. C、In summer. D、In winter. B

Life was tough. Life was still tough.(7)A long time ago, humans were only as tall as hens and rabbits. These tiny "humans" , if you can call them that, lived under trees. Living in trees was not very nice. During rains they got wet. In summers they sweated madly, and in winter they almost froze to ice. Life was tough.(8)So they started living in caves, just the way rabbits do. They thought that at least this way they will be dry in the rains. And in summer, they will be safe from the hot sun. But, often, the roofs fell upon them without warning. They would get trapped inside, unable to come out. Life was still tough.(9)Then the humans decided to make a tree house. They created an umbrella of branches and leaves and lived under them. The roof did not fall upon them. Life became a bit better.
    But the houses did not have any walls. People continued to feel the heat and cold. Finally, they learnt the trick of making walls. They made the first real houses. There was no doubt any more that it was a good life.(10)It became even better when the humans made houses close to each other so that they would never feel alone.