英语完型中的一道题

题目

英语完型中的一道题
It happened some years ago on a hot summer day in Florida. A little boy decided to go for a swim in the lake behind his house. In a hurry to dive into the cool water, he ran out of the back door, leaving behind his clothes as he went. He_____ into the water, not realizing that as he swam toward the middle of the lake, an alligator was swimming toward the bank. His mother, in the house, was looking out of the window and saw the two as they got closer and closer together. In great fear, she ran toward the water and shouted to him as loudly as she could.

alligator：短吻鳄
这个空有四个选项：
A. walked B. flew C. ran D. swam
答案是B,我要解释,详细明了的有加分

答案