下列过程放出热量的是（　　）

    从前，有位画师收了三个徒弟。

    有一天，画师把三个徒弟召集在一起，要考考他们。他给每个徒弟一张同样大小的纸，让他们画骆驼，看谁画的骆驼最多。

    徒弟们想了一会儿，便拿起笔画了起来。

    大徒弟用细笔密密麻麻地在纸上画满了很小很小的骆驼。画完以后，他很得意，以为自己画得最多。

    二徒弟想：纸只有这么大，要画出最多的骆驼，该怎么办呢？于是他画了许许多多骆驼的头。他画的果然比大徒弟多。

    画师看了看他俩的画，没有露出满意的神情。当他拿起小徒弟的画时，禁不住点头称赞。原来，小徒弟只画了几条弯弯曲曲的线，表示连绵不断的山峰，一只骆驼从山中走出来，另一只骆驼只露出脑袋和半截脖子。看到画师称赞小徒弟的画，大徒弟和二徒弟感到很奇怪。

    画师说：“你们看这幅画，画上虽然只有两只骆驼，但它们在连绵起伏的群山里走着，若隐若现，谁也说不清会从山谷里走出多少只骆驼，这不恰好表明有数不尽的骆驼吗？”

    两个徒弟恍然大悟。

已知x，y满足$\left\{\begin{array}{c}x+2y=7\\ 2x+y=2\end{array}\right.$，则x﹣y的值是____ ．

阅读理解

      Little Mike's grandma died weeks ago. He missed her very much. One afternoon Mike went to the city park. His grandma used to go there. There he saw an old lady. She looked very kind. She was sitting there, watching pigeons (鸽子). Little Mike went up and sat next to her. He took out his food and drinks, and gave some to her. She smiled at him .Her smile was very sweet. She seemed to understand him, and she smiled again. Mike was very glad.

       They sat there all the afternoon, eating and talking. As it grew dark, Mike got up to leave. Before he left, he hugged the old lady and she gave him her sweetest smile.

     When Mike got home, his mother was surprised by the look of joy on his face.“I met a granny in the park. Her smile was like the kind of my grandma's.” said Mike.

     The old lady also returned to her home happily. She told her son she had food and drinks with a little boy. “He was as lovely as Brittany.” She said. Her son was surprised, because he had never seen her so happy since Brittany, her grandson, died weeks ago.