如图，人的造血干细胞产生血细胞的过程在生物学上称为（　　）

阅读短文（A），用方框中词的适当形式填空，使文章通顺，连贯，合理（每空一词，每次限用一次）

A

A train ____ at a small station. A man looked out of the window and ____ a woman selling cakes. People from the train were ____ them. The man wanted to buy one, but the woman was standing far ____ the train. Suddenly he saw a boy walking past him. “Come here, boy!” the man said, “How much is the cake, do you know?” “One yuan for each,” the boy answered.

The man gave the boy two yuan and ____   him to buy two cakes. “One for me and the other for you.” he said to the boy. In a minute, the boy came back. He was eating a cake. He gave the man one yuan and said. “There are only one cake.”

中科院院士袁隆平在培育超级杂交水稻时，专门挑选叶片直、窄、厚的水稻植株，其目的是____。