Choose B. First of all, there is a question that I don't know if you can understand (this high school is easy to understand). The relationship between frequency and period is reciprocal. Frequency: how many times per unit time, period: how long does it take once,

So the driving cycle in the same direction is a divided by {v parent-v), and the frequency is {v parent-v) divided by a.

The reverse driving cycle is a divided by {v parent +v), and the frequency is {v parent-v) divided by a.

Then the equation is listed according to the condition of 1 1 times.