Why Do Electrons Move in Spiral Trajectories Within a Magnetic Field?

[S. Hardt]

Hello I do not understand one of the problems in my SAT Physics book.

Homework Statement

The subsequent path of the electron is...
.
the answer for this given by the book is : Spiral or Helix
(It does not exist any picture in the book to this question.)

The explanation the book gives for this answer is:
The electron will orbit a magnetic field line but will also continue to move towards the bottom of the page , spiraling downwards.

My question for this is Why is it moving that way? There is no explanation given.This question is out of the SAT Subject Test: Physics from Kaplan.

It would be nice to get an answer soon.
I am studying SAT Physics in a Club, so we all have got the same problem with this.

 

[S. Hardt]

Ah okay I figured out half of it... there was a picture...
The force applied on the proton is according to the Right hand rule into the page.

So the question left is only: why does a charged particle moves in a spiral into the page and not in a line (of course not in the beginning but after the direction changed).

 

[S. Hardt]

We figured out the answer so there is no reply needed anymore