the reason why the windshield wipers can\'t clean

In the rainy weather, which of the following is the reason why the windshield wipers cannot be cleaned as cleanly as the glass?

1- The air is cool  

2- The wiping of the wiper blades

3- Decrease of glass water level

4- Slow operation of the wiper motor


The right answer is: 2- The wiping of the wiper blades