为下列图片选择适当的句子：

设a是实数，函数f（x）=a﹣ $\frac{2}{2^x+1}$ （x∈R），

先通读下面的短文，掌握其大意，然后从A、B、C、D四个选项中选择可以填入空白处的最佳答案。
    Once there was an old woman who liked to make cookies(曲奇饼).She didn't have1to share them with，so she always gave them away to the children in the neighborhood.This continued for several years and the cookies became quite famous，2at Christmas time.The parents began to like the cookies as much as the children did，and the old woman's kitchen became very busy, so the children came to3.
Then one Christmas the old woman became ill.There were no cookies，and there was no money to pay the doctor.The children were very4.Later they had a(n)5 ， that is，to make cookies from the old woman's recipe(食谱).So the children 6a cookie store.Soon they had a very good cookie business and got7money to Pay the doctor.
    After the old woman got well，she was asked to become the manager(经理)of the store，selling special cookies8all the holidays of the year.9the old woman died，the children，now grown up，kept the cookie business alive(有生气的)in her name.
    Maybe life is like a Christmas cookie.The more you10it with your friends, the more you may enjoy it.

当细胞生长到一定大小时就会停止生长，有些细胞可以____成为两个____ 的新细胞，这就是____ ．

       the light red?