Calculating Period of Transverse Wave from Photo: A or B?

[tomcps]

Homework Statement

The problem is shown on the photo. And the actual answer is A. 0.5s, and I thought it would be B. 1.0s

Homework Equations

The Attempt at a Solution

Here is my thought,
The journey from A to B is just a half period, then the whole period would be 4s, as a result, the time it takes for C to return would be 4÷4= 1.0s
Isn't it? Sorry i don't have good concepts of wave

 

[BvU]

Hi tom,

I agree with your answer. The actual answer given must be a mistake

 

[tomcps]

Hi tom,

I agree with your answer. The actual answer given must be a mistake

Really? I am so confused now