The simulation shows a sequence of positive charges +q flowing to the right with a speed v. Which of the following corresponds to a doubling of the current?

1. Twice as many charges going right at v
2. Same number of charges going right at 2v
3. Add -q charges going right at v
4. Add -q charges going left at v
5. Both 1 and 2
6. 1, 2 and 3
7. 1, 2 and 4
8. 1 and 3
9. 1 and 4

Choice 3 corresponds to zero current. Choices 1, 2, and 4 all correspond to a doubling of the current. What matters is the number of charges that pass a particular point in a given time interval. Also, +q charges flowing one way are equivalent to -q charges flowing in the opposite direction.