Last summer, Jeff became a middle school student. He was a good student, and he always worked hard. He had a test in the first month and he finished the questions easily until he read the last one:”小题1: What’s her name?”

Of course, this question was a little special.   小题2:She was tall and her hair was dark. But Jeff didn’t talk with her and didn’t know her name. The test was over.   小题3: Then a student asked the teacher, “Is the last question very important in this test?”

“Yes,” said the teacher. “In our life, we will meet many hard-working people. They are important.  小题4: We may smile at them or say hello to them. 

 小题5: He also learned that woman’s name later. It was Kathy.

根据材料内容，从下面五个选项中选出能填入文中空缺处的最佳选项，使短文意思通顺，内容完整，并将其标号填写在下面题号后的横线上。

一艘船沿河顺水而行的航速为30千米/小时，已知按同样的航速在该河上顺水航行3小时和逆水航行5小时的航程相等，则此船在该河上顺水漂流半小时的航程为( )。

A．1千米

B．2千米

C．3千米

D．6千米