vector product between $ds$ and other portion of current loop

In the answer section it is written that,The magnetic field at O due to the current in the straight segments $AA^\prime$ and $CC^\prime$ is zero because ds is parallel to along these paths.

My querry is that "How ds is parallel to along these paths?"

EDIT:

• The magnetic field at "O" due to current in AA' and CC' is zero irrespective of ds - where did you get the quote? – Andy aka Nov 3 '13 at 17:35
• Resnick Hallyday modern physics page no 941. Iam adding the screenshot. – user2378 Nov 3 '13 at 17:52

In your question you probably mistakingly omited the $\hat{r}$ part which is why I asked for clarification.
• But why ds is paral- lel to $\hat r$ – user2378 Nov 3 '13 at 18:40
• $\hat{r}$ is a vector produced by current and always points at O. When ds is on the section AA' or CC' $\hat{r}$ still points at O but it is now in parallel with ds. – Andy aka Nov 3 '13 at 18:46