Particle detectors and Flemming LH rule

[jsmith613]

Homework Statement

http://www.thestudentroom.co.uk/attachment.php?attachmentid=118892&d=1329155852

(the most important aspect is the CHARGE)

Homework Equations

The Attempt at a Solution

so we know that the first finger (using flemming LHR) is into the page
but there are two possibilities:
(a) current up and force left
OR
(b) current right and force up

WHICH one would I use and why?
I have never really been able to distinguish the two from each other with regards to LH rule in questions like this

thanks

 

[jsmith613]

btw I think C is +ive
who agrees?